Merge pull request #1834 from molstar/fix-aromatic-ring-hybridization

Fix aromatic ring detection not accounting for hybridization

CHANGELOG.md

@@ -16,7 +16,10 @@ Note that since we don't clearly distinguish between a public and private interf
 - Fix memory leak in `State.dispose()` not invoking transformer `dispose` callbacks for live cells
 - Fix bugs in ModelServer surroundingLigands endpoint, resulting in omitWater not honored
 - Fix `Volume` and `Isosurface` getBoundingSphere ignoring instances
+- Fix aromatic ring detection not accounting for hybridization
+- Add axis param to camera spin/rock animation
 - Fix SSAO half/quarter resolution textures for multi-scale
+- Non-covalent interactions: water bridge support
 
 ## [v5.9.0] - 2026-05-03
 - Fix edge case when `PluginSpec.animations` is empty

src/extensions/interactions/model.ts

@@ -25,6 +25,7 @@ export type InteractionElementSchema =
     | { kind: 'weak-hydrogen-bond' } & InteractionElementSchemaBase
     | { kind: 'hydrophobic' } & InteractionElementSchemaBase
     | { kind: 'metal-coordination' } & InteractionElementSchemaBase
+    | { kind: 'water-bridge' } & InteractionElementSchemaBase
     | { kind: 'covalent', degree?: 'aromatic' | 1 | 2 | 3 | 4 } & InteractionElementSchemaBase
 
 export type InteractionKind = InteractionElementSchema['kind']
@@ -39,6 +40,7 @@ export const InteractionKinds: InteractionKind[] = [
     'weak-hydrogen-bond',
     'hydrophobic',
     'metal-coordination',
+    'water-bridge',
     'covalent',
 ];
 
@@ -52,6 +54,7 @@ export type InteractionInfo =
     | { kind: 'weak-hydrogen-bond', hydrogenStructureRef?: string, hydrogen?: StructureElement.Loci }
     | { kind: 'hydrophobic' }
     | { kind: 'metal-coordination' }
+    | { kind: 'water-bridge' }
     | { kind: 'covalent', degree?: 'aromatic' | 1 | 2 | 3 | 4 }
 
 export interface StructureInteractionElement {
@@ -80,4 +83,5 @@ export const InteractionTypeToKind = {
     [InteractionType.Hydrophobic]: 'hydrophobic' as InteractionKind,
     [InteractionType.MetalCoordination]: 'metal-coordination' as InteractionKind,
     [InteractionType.WeakHydrogenBond]: 'weak-hydrogen-bond' as InteractionKind,
+    [InteractionType.WaterBridge]: 'water-bridge' as InteractionKind,
 };

src/extensions/interactions/visuals.ts

@@ -47,6 +47,7 @@ export const InteractionVisualParams = {
         'weak-hydrogen-bond': hydrogenVisualParams({ color: Color(0x0) }),
         'hydrophobic': visualParams({ color: Color(0x555555) }),
         'metal-coordination': visualParams({ color: Color(0x952e8f) }),
+        'water-bridge': visualParams({ color: Color(0x00CCEE), style: 'dashed' }),
         'covalent': PD.Group({
             color: PD.Color(Color(0x999999)),
             radius: PD.Numeric(0.1, { min: 0.01, max: 1, step: 0.01 }),

src/mol-model-props/computed/interactions/common.ts

@@ -133,6 +133,7 @@ export enum InteractionType {
     Hydrophobic = 6,
     MetalCoordination = 7,
     WeakHydrogenBond = 8,
+    WaterBridge = 9,
 }
 
 export function interactionTypeLabel(type: InteractionType): string {
@@ -153,6 +154,8 @@ export function interactionTypeLabel(type: InteractionType): string {
             return 'Pi Stacking';
         case InteractionType.WeakHydrogenBond:
             return 'Weak Hydrogen Bond';
+        case InteractionType.WaterBridge:
+            return 'Water Bridge';
         case InteractionType.Unknown:
             return 'Unknown Interaction';
     }

src/mol-model-props/computed/interactions/hydrogen-bonds.ts

@@ -20,7 +20,7 @@ import { FeatureType, FeatureGroup, InteractionType } from './common';
 import { ContactProvider } from './contacts';
 import { MoleculeType, ProteinBackboneAtoms } from '../../../mol-model/structure/model/types';
 
-const GeometryParams = {
+export const GeometryParams = {
     distanceMax: PD.Numeric(3.5, { min: 1, max: 5, step: 0.1 }),
     backbone: PD.Boolean(true, { description: 'Include backbone-to-backbone hydrogen bonds' }),
     accAngleDevMax: PD.Numeric(45, { min: 0, max: 180, step: 1 }, { description: 'Max deviation from ideal acceptor angle' }),
@@ -29,7 +29,7 @@ const GeometryParams = {
     accOutOfPlaneAngleMax: PD.Numeric(90, { min: 0, max: 180, step: 1 }),
     donOutOfPlaneAngleMax: PD.Numeric(45, { min: 0, max: 180, step: 1 }),
 };
-type GeometryParams = typeof GeometryParams
+export type GeometryParams = typeof GeometryParams
 type GeometryProps = PD.Values<GeometryParams>
 
 const HydrogenBondsParams = {
@@ -208,7 +208,7 @@ function isWeakHydrogenBond(ti: FeatureType, tj: FeatureType) {
     );
 }
 
-function getGeometryOptions(props: GeometryProps) {
+export function getGeometryOptions(props: GeometryProps) {
     return {
         ignoreHydrogens: props.ignoreHydrogens,
         includeBackbone: props.backbone,
@@ -218,7 +218,7 @@ function getGeometryOptions(props: GeometryProps) {
         maxDonOutOfPlaneAngle: degToRad(props.donOutOfPlaneAngleMax),
     };
 }
-type GeometryOptions = ReturnType<typeof getGeometryOptions>
+export type GeometryOptions = ReturnType<typeof getGeometryOptions>
 
 function getHydrogenBondsOptions(props: HydrogenBondsProps) {
     return {
@@ -232,7 +232,7 @@ type HydrogenBondsOptions = ReturnType<typeof getHydrogenBondsOptions>
 
 const deg120InRad = degToRad(120);
 
-function checkGeometry(structure: Structure, don: Features.Info, acc: Features.Info, opts: GeometryOptions): true | undefined {
+export function checkGeometry(structure: Structure, don: Features.Info, acc: Features.Info, opts: GeometryOptions): true | undefined {
     const donIndex = don.members[don.offsets[don.feature]];
     const accIndex = acc.members[acc.offsets[acc.feature]];
 

src/mol-model-props/computed/interactions/interactions.ts

@@ -1,20 +1,21 @@
 /**
- * Copyright (c) 2019-2025 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2019-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  * @author David Sehnal <david.sehnal@gmail.com>
  */
 
 import { ParamDefinition as PD } from '../../../mol-util/param-definition';
-import { Structure, Unit, Bond } from '../../../mol-model/structure';
+import { Structure, Unit, Bond, StructureElement } from '../../../mol-model/structure';
 import { Features, FeaturesBuilder } from './features';
 import { ValenceModelProvider } from '../valence-model';
-import { InteractionsIntraContacts, InteractionsInterContacts, FeatureType, interactionTypeLabel } from './common';
+import { InteractionsIntraContacts, InteractionsInterContacts, FeatureType, InteractionType, InteractionFlag, interactionTypeLabel } from './common';
 import { IntraContactsBuilder, InterContactsBuilder } from './contacts-builder';
-import { IntMap } from '../../../mol-data/int';
+import { IntMap, OrderedSet } from '../../../mol-data/int';
 import { addUnitContacts, ContactTester, addStructureContacts, ContactsParams, ContactsProps } from './contacts';
 import { HalogenDonorProvider, HalogenAcceptorProvider, HalogenBondsProvider } from './halogen-bonds';
 import { HydrogenDonorProvider, WeakHydrogenDonorProvider, HydrogenAcceptorProvider, HydrogenBondsProvider, WeakHydrogenBondsProvider } from './hydrogen-bonds';
+import { WaterBridgesProvider } from './water-bridges';
 import { NegativChargeProvider, PositiveChargeProvider, AromaticRingProvider, IonicProvider, PiStackingProvider, CationPiProvider } from './charged';
 import { HydrophobicAtomProvider, HydrophobicProvider } from './hydrophobic';
 import { SetUtils } from '../../../mol-util/set';
@@ -25,10 +26,26 @@ import { DataLocation } from '../../../mol-model/location';
 import { CentroidHelper } from '../../../mol-math/geometry/centroid-helper';
 import { Sphere3D } from '../../../mol-math/geometry';
 import { DataLoci } from '../../../mol-model/loci';
-import { bondLabel, LabelGranularity } from '../../../mol-theme/label';
+import { bondLabel, bundleLabel, LabelGranularity } from '../../../mol-theme/label';
 import { ObjectKeys } from '../../../mol-util/type-helpers';
 
-export { Interactions };
+export { Interactions, Bridges };
+export type { BridgeContact, BridgeContacts };
+
+interface BridgeContact {
+    readonly unitA: number
+    readonly indexA: Features.FeatureIndex
+    readonly unitB: number
+    readonly indexB: Features.FeatureIndex
+    /** mediator unit id */
+    readonly unitM: number
+    /** mediator feature facing endpoint A */
+    readonly indexMA: Features.FeatureIndex
+    /** mediator feature facing endpoint B */
+    readonly indexMB: Features.FeatureIndex
+    props: { type: InteractionType, flag: InteractionFlag }
+}
+type BridgeContacts = ReadonlyArray<BridgeContact>
 
 interface Interactions {
     /** Features of each unit */
@@ -37,6 +54,8 @@ interface Interactions {
     unitsContacts: IntMap<InteractionsIntraContacts>
     /** Interactions between units */
     contacts: InteractionsInterContacts
+    /** Bridge-mediated interactions covering the whole structure */
+    bridges: BridgeContacts
 }
 
 namespace Interactions {
@@ -129,6 +148,93 @@ namespace Interactions {
     }
 }
 
+namespace Bridges {
+    export interface Data {
+        readonly structure: Structure
+        readonly bridges: BridgeContacts
+        readonly unitsFeatures: IntMap<Features>
+    }
+
+    export interface Element { bridgeIndex: number }
+
+    export interface Location extends DataLocation<Data, Element> {}
+
+    export function Location(data: Data, bridgeIndex = 0): Location {
+        return DataLocation('bridges', data, { bridgeIndex });
+    }
+
+    export function isLocation(x: any): x is Location {
+        return !!x && x.kind === 'data-location' && x.tag === 'bridges';
+    }
+
+    export interface Loci extends DataLoci<Data, Element> {}
+
+    export function Loci(data: Data, elements: ReadonlyArray<Element>): Loci {
+        return DataLoci('bridges', data, elements,
+            bs => getBoundingSphere(data, elements, bs),
+            () => getLabel(data, elements));
+    }
+
+    export function isLoci(x: any): x is Loci {
+        return !!x && x.kind === 'data-loci' && x.tag === 'bridges';
+    }
+
+    function getLabel(data: Data, elements: ReadonlyArray<Element>): string {
+        const e = elements[0];
+        if (e === undefined) return '';
+
+        const { structure, bridges, unitsFeatures } = data;
+        const bridge = bridges[e.bridgeIndex];
+
+        const uA = structure.unitMap.get(bridge.unitA) as Unit.Atomic;
+        const fA = unitsFeatures.get(bridge.unitA);
+        const uM = structure.unitMap.get(bridge.unitM) as Unit.Atomic;
+        const fM = unitsFeatures.get(bridge.unitM);
+        const uB = structure.unitMap.get(bridge.unitB) as Unit.Atomic;
+        const fB = unitsFeatures.get(bridge.unitB);
+
+        const options = { granularity: 'element' as LabelGranularity };
+        if (fA.offsets[bridge.indexA + 1] - fA.offsets[bridge.indexA] > 1 ||
+                fB.offsets[bridge.indexB + 1] - fB.offsets[bridge.indexB] > 1) {
+            options.granularity = 'residue';
+        }
+
+        return [
+            interactionTypeLabel(bridge.props.type),
+            bundleLabel({ loci: [
+                StructureElement.Loci(structure, [{ unit: uA, indices: OrderedSet.ofSingleton(fA.members[fA.offsets[bridge.indexA]] as StructureElement.UnitIndex) }]),
+                StructureElement.Loci(structure, [{ unit: uM, indices: OrderedSet.ofSingleton(fM.members[fM.offsets[bridge.indexMA]] as StructureElement.UnitIndex) }]),
+                StructureElement.Loci(structure, [{ unit: uB, indices: OrderedSet.ofSingleton(fB.members[fB.offsets[bridge.indexB]] as StructureElement.UnitIndex) }]),
+            ] }, options),
+        ].join('</br>');
+    }
+
+    function getBoundingSphere(data: Data, elements: ReadonlyArray<Element>, boundingSphere: Sphere3D) {
+        return CentroidHelper.fromPairProvider(elements.length * 2, (i, pA, pB) => {
+            const bridge = data.bridges[elements[i >> 1].bridgeIndex];
+
+            const uA = data.structure.unitMap.get(bridge.unitA) as Unit.Atomic;
+            const fA = data.unitsFeatures.get(bridge.unitA);
+            const uM = data.structure.unitMap.get(bridge.unitM) as Unit.Atomic;
+            const fM = data.unitsFeatures.get(bridge.unitM);
+            const uB = data.structure.unitMap.get(bridge.unitB) as Unit.Atomic;
+            const fB = data.unitsFeatures.get(bridge.unitB);
+
+            const aIdx = fA.members[fA.offsets[bridge.indexA]];
+            const mIdx = fM.members[fM.offsets[bridge.indexMA]];
+            const bIdx = fB.members[fB.offsets[bridge.indexB]];
+
+            if ((i & 1) === 0) {
+                uA.conformation.position(uA.elements[aIdx], pA);
+                uM.conformation.position(uM.elements[mIdx], pB);
+            } else {
+                uM.conformation.position(uM.elements[mIdx], pA);
+                uB.conformation.position(uB.elements[bIdx], pB);
+            }
+        }, boundingSphere);
+    }
+}
+
 const FeatureProviders = [
     HydrogenDonorProvider, WeakHydrogenDonorProvider, HydrogenAcceptorProvider,
     NegativChargeProvider, PositiveChargeProvider, AromaticRingProvider,
@@ -174,8 +280,30 @@ export const ContactProviderParams = getProvidersParams([
     // 'weak-hydrogen-bonds',
 ]);
 
+const BridgeProviders = {
+    'water-bridges': WaterBridgesProvider,
+};
+type BridgeProviders = typeof BridgeProviders
+
+function getBridgeProviderParams(defaultOn: string[] = []) {
+    const params: { [k in keyof BridgeProviders]: PD.Mapped<PD.NamedParamUnion<{
+        on: PD.Group<BridgeProviders[k]['params']>
+        off: PD.Group<{}>
+    }>> } = Object.create(null);
+
+    Object.keys(BridgeProviders).forEach(k => {
+        (params as any)[k] = PD.MappedStatic(defaultOn.includes(k) ? 'on' : 'off', {
+            on: PD.Group(BridgeProviders[k as keyof BridgeProviders].params),
+            off: PD.Group({})
+        }, { cycle: true });
+    });
+    return params;
+}
+export const BridgeProviderParams = getBridgeProviderParams([]);
+
 export const InteractionsParams = {
     providers: PD.Group(ContactProviderParams, { isFlat: true }),
+    bridges: PD.Group(BridgeProviderParams, { isFlat: true }),
     contacts: PD.Group(ContactsParams, { label: 'Advanced Options' }),
 };
 export type InteractionsParams = typeof InteractionsParams
@@ -202,6 +330,9 @@ export async function computeInteractions(ctx: CustomProperty.Context, structure
 
     const requiredFeatures = new Set<FeatureType>();
     contactTesters.forEach(l => SetUtils.add(requiredFeatures, l.requiredFeatures));
+    ObjectKeys(BridgeProviders).forEach(k => {
+        if (p.bridges[k].name === 'on') SetUtils.add(requiredFeatures, BridgeProviders[k].requiredFeatures);
+    });
     const featureProviders = FeatureProviders.filter(f => SetUtils.areIntersecting(requiredFeatures, f.types));
 
     const unitsFeatures = IntMap.Mutable<Features>();
@@ -228,8 +359,9 @@ export async function computeInteractions(ctx: CustomProperty.Context, structure
     }
 
     const contacts = findInterUnitContacts(structure, unitsFeatures, contactTesters, p.contacts, options);
+    const bridges = findBridges(structure, unitsFeatures, p.bridges);
+    const interactions = { unitsFeatures, unitsContacts, contacts, bridges };
 
-    const interactions = { unitsFeatures, unitsContacts, contacts };
     refineInteractions(structure, interactions);
     return interactions;
 }
@@ -260,6 +392,19 @@ function findIntraUnitContacts(structure: Structure, unit: Unit, features: Featu
     return builder.getContacts();
 }
 
+function findBridges(structure: Structure, unitsFeatures: IntMap<Features>, props: PD.Values<typeof BridgeProviderParams>): BridgeContacts {
+    const bridges: BridgeContact[] = [];
+
+    ObjectKeys(BridgeProviders).forEach(k => {
+        const { name, params } = props[k];
+        if (name === 'on') {
+            for (const b of BridgeProviders[k].find(structure, unitsFeatures, params as any)) bridges.push(b);
+        }
+    });
+
+    return bridges;
+}
+
 function findInterUnitContacts(structure: Structure, unitsFeatures: IntMap<Features>, contactTesters: ReadonlyArray<ContactTester>, props: ContactsProps, options?: ComputeInterctionsOptions) {
     const builder = InterContactsBuilder.create();
 

src/mol-model-props/computed/interactions/refine.ts

@@ -1,15 +1,17 @@
 /**
- * Copyright (c) 2019-2020 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2019-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
+ * @author Sebastian Bittrich <sebastian.m.bittrich@gmail.com>
  *
  * based in part on NGL (https://github.com/arose/ngl)
  */
 
 import { Interactions } from './interactions';
 import { InteractionType, InteractionFlag, InteractionsIntraContacts, FeatureType, InteractionsInterContacts } from './common';
-import { Unit, Structure } from '../../../mol-model/structure';
+import { Unit, Structure, StructureElement } from '../../../mol-model/structure';
 import { Features } from './features';
+import { cantorPairing } from '../../../mol-data/util/hash-functions';
 
 interface ContactRefiner {
     isApplicable: (type: InteractionType) => boolean
@@ -27,6 +29,7 @@ export function refineInteractions(structure: Structure, interactions: Interacti
         saltBridgeRefiner(structure, interactions),
         piStackingRefiner(structure, interactions),
         metalCoordinationRefiner(structure, interactions),
+        waterBridgeRefiner(structure, interactions),
     ];
 
     for (let i = 0, il = contacts.edgeCount; i < il; ++i) {
@@ -278,4 +281,117 @@ function metalCoordinationRefiner(structure: Structure, interactions: Interactio
             filterIntra([InteractionType.MetalCoordination], index, infoA, infoB, interactions.unitsContacts.get(infoA.unit.id));
         }
     };
+}
+
+function waterBridgeRefiner(_structure: Structure, interactions: Interactions): ContactRefiner {
+    const { contacts, bridges, unitsFeatures } = interactions;
+
+    type AtomKey = number;
+    type AtomPairSet = Map<AtomKey, Set<AtomKey>>;
+
+    function atomKey(unitId: number, atomIndex: StructureElement.UnitIndex): AtomKey {
+        return cantorPairing(unitId, atomIndex);
+    }
+
+    function featureMember(features: Features, featureIndex: Features.FeatureIndex): StructureElement.UnitIndex {
+        return features.members[features.offsets[featureIndex]] as StructureElement.UnitIndex;
+    }
+
+    function addAtomPair(
+        set: AtomPairSet,
+        unitA: number,
+        atomA: StructureElement.UnitIndex,
+        unitB: number,
+        atomB: StructureElement.UnitIndex
+    ) {
+        const a = atomKey(unitA, atomA);
+        const b = atomKey(unitB, atomB);
+
+        let bs = set.get(a);
+        if (bs === undefined) {
+            bs = new Set();
+            set.set(a, bs);
+        }
+        bs.add(b);
+
+        let as = set.get(b);
+        if (as === undefined) {
+            as = new Set();
+            set.set(b, as);
+        }
+        as.add(a);
+    }
+
+    function hasAtomPair(
+        set: AtomPairSet,
+        unitA: number,
+        atomA: StructureElement.UnitIndex,
+        unitB: number,
+        atomB: StructureElement.UnitIndex
+    ): boolean {
+        return set.get(atomKey(unitA, atomA))?.has(atomKey(unitB, atomB)) === true;
+    }
+
+    function hasInfoPair(set: AtomPairSet, infoA: Features.Info, infoB: Features.Info): boolean {
+        const { offsets: offsetsA, members: membersA, feature: featureA } = infoA;
+        const { offsets: offsetsB, members: membersB, feature: featureB } = infoB;
+
+        for (let i = offsetsA[featureA], il = offsetsA[featureA + 1]; i < il; ++i) {
+            const a = membersA[i] as StructureElement.UnitIndex;
+
+            for (let j = offsetsB[featureB], jl = offsetsB[featureB + 1]; j < jl; ++j) {
+                const b = membersB[j] as StructureElement.UnitIndex;
+
+                if (hasAtomPair(set, infoA.unit.id, a, infoB.unit.id, b)) return true;
+            }
+        }
+
+        return false;
+    }
+
+    const bridgeLegs: AtomPairSet = new Map();
+
+    for (const wb of bridges) {
+        if (wb.props.type !== InteractionType.WaterBridge) continue;
+
+        const fA = unitsFeatures.get(wb.unitA);
+        const fM = unitsFeatures.get(wb.unitM);
+        const fB = unitsFeatures.get(wb.unitB);
+
+        if (!fA || !fM || !fB) continue;
+
+        const atomA = featureMember(fA, wb.indexA);
+        const atomMA = featureMember(fM, wb.indexMA);
+        const atomMB = featureMember(fM, wb.indexMB);
+        const atomB = featureMember(fB, wb.indexB);
+
+        // donor atom ↔ water oxygen
+        addAtomPair(bridgeLegs, wb.unitA, atomA, wb.unitM, atomMA);
+
+        // water oxygen ↔ acceptor atom
+        addAtomPair(bridgeLegs, wb.unitM, atomMB, wb.unitB, atomB);
+    }
+
+    let intraContacts: InteractionsIntraContacts | undefined;
+
+    return {
+        isApplicable: (type: InteractionType) => {
+            return bridgeLegs.size > 0 && type === InteractionType.HydrogenBond;
+        },
+        handleInterContact: (index: number, infoA: Features.Info, infoB: Features.Info) => {
+            if (hasInfoPair(bridgeLegs, infoA, infoB)) {
+                contacts.edges[index].props.flag = InteractionFlag.Filtered;
+            }
+        },
+        startUnit: (_unit: Unit.Atomic, contacts: InteractionsIntraContacts) => {
+            intraContacts = contacts;
+        },
+        handleIntraContact: (index: number, infoA: Features.Info, infoB: Features.Info) => {
+            if (!intraContacts) return;
+
+            if (hasInfoPair(bridgeLegs, infoA, infoB)) {
+                intraContacts.edgeProps.flag[index] = InteractionFlag.Filtered;
+            }
+        },
+    };
 }

src/mol-model-props/computed/interactions/water-bridges.ts

@@ -0,0 +1,331 @@
+/**
+ * Copyright (c) 2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ *
+ * @author Sebastian Bittrich <sebastian.m.bittrich@gmail.com>
+ */
+
+import { Structure, Unit, StructureElement } from '../../../mol-model/structure';
+import { IntMap } from '../../../mol-data/int';
+import { Vec3 } from '../../../mol-math/linear-algebra';
+import { ParamDefinition as PD } from '../../../mol-util/param-definition';
+import { MoleculeType, NucleicBackboneAtoms, ProteinBackboneAtoms } from '../../../mol-model/structure/model/types';
+import { StructureLookup3DResultContext } from '../../../mol-model/structure/structure/util/lookup3d';
+import { Features } from './features';
+import { FeatureType, InteractionType, InteractionFlag } from './common';
+import { GeometryOptions, checkGeometry } from './hydrogen-bonds';
+import { degToRad } from '../../../mol-math/misc';
+import { cantorPairing } from '../../../mol-data/util/hash-functions';
+
+export type { WaterBridgeContact, WaterBridgeContacts };
+
+interface WaterBridgeContact {
+    /** non-water donor unit id */
+    readonly unitA: number
+    /** donor feature index in unitA */
+    readonly indexA: Features.FeatureIndex
+    /** non-water acceptor unit id */
+    readonly unitB: number
+    /** acceptor feature index in unitB */
+    readonly indexB: Features.FeatureIndex
+    /** bridging water unit id */
+    readonly unitM: number
+    /** water oxygen as HydrogenAcceptor (leg: donor → water) */
+    readonly indexMA: Features.FeatureIndex
+    /** water oxygen as HydrogenDonor (leg: water → acceptor) */
+    readonly indexMB: Features.FeatureIndex
+    props: { type: InteractionType.WaterBridge, flag: InteractionFlag }
+}
+
+type WaterBridgeContacts = ReadonlyArray<WaterBridgeContact>;
+
+export const WaterBridgesParams = {
+    backbone: PD.Boolean(true, { description: 'Include backbone hydrogen bonds' }),
+    ignoreHydrogens: PD.Boolean(true, { description: 'Ignore explicit hydrogens in geometric constraints' }),
+    legDistMin: PD.Numeric(2.5, { min: 1, max: 4, step: 0.1 }, { description: 'Minimum leg distance (Å)' }),
+    legDistMax: PD.Numeric(4.1, { min: 1, max: 6, step: 0.1 }, { description: 'Maximum leg distance (Å)' }),
+    donAngleDevMax: PD.Numeric(80, { min: 0, max: 180, step: 1 }, { description: 'Max deviation from ideal donor angle' }),
+    accAngleDevMax: PD.Numeric(50, { min: 0, max: 180, step: 1 }, { description: 'Max deviation from ideal acceptor angle' }),
+    donOutOfPlaneAngleMax: PD.Numeric(45, { min: 0, max: 180, step: 1 }),
+    accOutOfPlaneAngleMax: PD.Numeric(90, { min: 0, max: 180, step: 1 }),
+    omegaMin: PD.Numeric(71, { min: 0, max: 180, step: 1 }, { description: 'Minimum A–W–B angle (°)' }),
+    omegaMax: PD.Numeric(140, { min: 0, max: 180, step: 1 }, { description: 'Maximum A–W–B angle (°)' }),
+};
+export type WaterBridgesParams = typeof WaterBridgesParams;
+export type WaterBridgesProps = PD.Values<WaterBridgesParams>;
+
+export const WaterBridgesProvider = {
+    requiredFeatures: new Set([FeatureType.HydrogenDonor, FeatureType.HydrogenAcceptor]),
+    params: WaterBridgesParams,
+    find: findWaterBridgeContacts,
+};
+
+function isWater(unit: Unit.Atomic, index: StructureElement.UnitIndex): boolean {
+    return unit.model.atomicHierarchy.derived.residue.moleculeType[
+        unit.residueIndex[unit.elements[index]]
+        ] === MoleculeType.Water;
+}
+
+function isBackboneAtom(unit: Unit.Atomic, index: StructureElement.UnitIndex): boolean {
+    const element = unit.elements[index];
+    const moleculeType = unit.model.atomicHierarchy.derived.residue.moleculeType[unit.residueIndex[element]];
+    if (moleculeType !== MoleculeType.Protein && moleculeType !== MoleculeType.RNA && moleculeType !== MoleculeType.DNA) {
+        return false;
+    }
+
+    const atomId = unit.model.atomicHierarchy.atoms.label_atom_id.value(element);
+    if (moleculeType === MoleculeType.Protein) {
+        return ProteinBackboneAtoms.has(atomId);
+    }
+
+    return NucleicBackboneAtoms.has(atomId);
+}
+
+const _lookupCtx = StructureLookup3DResultContext();
+
+type Candidate = {
+    unit: Unit.Atomic
+    featureIdx: Features.FeatureIndex
+    memberIdx: StructureElement.UnitIndex
+    x: number
+    y: number
+    z: number
+    distSq: number
+};
+
+type FeatureKey = number;
+
+function featureKey(unitId: number, featureIndex: Features.FeatureIndex): FeatureKey {
+    return cantorPairing(unitId, featureIndex);
+}
+
+type BestBridge = { contact: WaterBridgeContact; combinedDistSq: number };
+type BestBridgeMap = Map<FeatureKey, Map<FeatureKey, BestBridge>>;
+
+function getBestBridge(best: BestBridgeMap, donorKey: FeatureKey, acceptorKey: FeatureKey): BestBridge | undefined {
+    return best.get(donorKey)?.get(acceptorKey);
+}
+
+function setBestBridge(best: BestBridgeMap, donorKey: FeatureKey, acceptorKey: FeatureKey, value: BestBridge) {
+    let acceptors = best.get(donorKey);
+    if (acceptors === undefined) {
+        acceptors = new Map();
+        best.set(donorKey, acceptors);
+    }
+    acceptors.set(acceptorKey, value);
+}
+
+function bestBridgeValues(best: BestBridgeMap): BestBridge[] {
+    const values: BestBridge[] = [];
+    for (const acceptors of best.values()) {
+        for (const value of acceptors.values()) values.push(value);
+    }
+    return values;
+}
+
+function checkOmega(don: Candidate, posW: Vec3, acc: Candidate, cosOmegaMin: number, cosOmegaMax: number): boolean {
+    const ax = don.x - posW[0];
+    const ay = don.y - posW[1];
+    const az = don.z - posW[2];
+
+    const bx = acc.x - posW[0];
+    const by = acc.y - posW[1];
+    const bz = acc.z - posW[2];
+
+    const aLenSq = ax * ax + ay * ay + az * az;
+    const bLenSq = bx * bx + by * by + bz * bz;
+
+    if (aLenSq === 0 || bLenSq === 0) return false;
+
+    const cosOmega = (ax * bx + ay * by + az * bz) / Math.sqrt(aLenSq * bLenSq);
+
+    // cos decreases monotonically on [0, pi], so:
+    // omega >= omegaMin && omega <= omegaMax
+    // is equivalent to:
+    // cos(omega) <= cos(omegaMin) && cos(omega) >= cos(omegaMax)
+    return cosOmega <= cosOmegaMin && cosOmega >= cosOmegaMax;
+}
+
+export function findWaterBridgeContacts(
+    structure: Structure,
+    unitsFeatures: IntMap<Features>,
+    props: WaterBridgesProps
+): WaterBridgeContacts {
+    const legOpts: GeometryOptions = {
+        ignoreHydrogens: props.ignoreHydrogens,
+        includeBackbone: props.backbone,
+        maxAccAngleDev: degToRad(props.accAngleDevMax),
+        maxDonAngleDev: degToRad(props.donAngleDevMax),
+        maxAccOutOfPlaneAngle: degToRad(props.accOutOfPlaneAngleMax),
+        maxDonOutOfPlaneAngle: degToRad(props.donOutOfPlaneAngleMax),
+    };
+
+    const legDistMinSq = props.legDistMin * props.legDistMin;
+    const legDistMaxSq = props.legDistMax * props.legDistMax;
+
+    const omegaMinRad = degToRad(props.omegaMin);
+    const omegaMaxRad = degToRad(props.omegaMax);
+
+    if (omegaMinRad > omegaMaxRad) return [];
+
+    const cosOmegaMin = Math.cos(omegaMinRad);
+    const cosOmegaMax = Math.cos(omegaMaxRad);
+
+    // Best bridge per unique donor/acceptor feature pair across all water molecules.
+    const best: BestBridgeMap = new Map();
+
+    const wPos = Vec3();
+    const candidatePos = Vec3();
+
+    for (const unitW of structure.units) {
+        if (!Unit.isAtomic(unitW)) continue;
+
+        const featW = unitsFeatures.get(unitW.id);
+        if (!featW || featW.count === 0) continue;
+
+        // Map each water-oxygen local index to its acceptor and donor feature indices.
+        const waterMap = new Map<StructureElement.UnitIndex, {
+            acc: Features.FeatureIndex | undefined,
+            don: Features.FeatureIndex | undefined
+        }>();
+
+        for (let fi = 0 as Features.FeatureIndex; fi < featW.count; fi++) {
+            const mi = featW.members[featW.offsets[fi]] as StructureElement.UnitIndex;
+            if (!isWater(unitW, mi)) continue;
+
+            const t = featW.types[fi];
+            if (t !== FeatureType.HydrogenAcceptor && t !== FeatureType.HydrogenDonor) continue;
+
+            let e = waterMap.get(mi);
+            if (!e) waterMap.set(mi, (e = { acc: undefined, don: undefined }));
+
+            if (t === FeatureType.HydrogenAcceptor) e.acc = fi;
+            else e.don = fi;
+        }
+
+        if (waterMap.size === 0) continue;
+
+        const infoWAcc = Features.Info(structure, unitW, featW);
+        const infoWDon = Features.Info(structure, unitW, featW);
+
+        for (const [waterAtomIdx, { acc: accFW, don: donFW }] of waterMap) {
+            if (accFW === undefined || donFW === undefined) continue;
+
+            unitW.conformation.position(unitW.elements[waterAtomIdx], wPos);
+
+            infoWAcc.feature = accFW;
+            infoWDon.feature = donFW;
+
+            const { count, indices, units: hitUnits } =
+                structure.lookup3d.find(wPos[0], wPos[1], wPos[2], props.legDistMax, _lookupCtx);
+
+            const donors: Candidate[] = [];
+            const acceptors: Candidate[] = [];
+
+            const donorKeys = new Set<FeatureKey>();
+            const acceptorKeys = new Set<FeatureKey>();
+
+            for (let r = 0; r < count; r++) {
+                const hitUnit = hitUnits[r];
+                if (!Unit.isAtomic(hitUnit)) continue;
+
+                const atomicUnit = hitUnit as Unit.Atomic;
+                const hitLocalIdx = indices[r] as StructureElement.UnitIndex;
+
+                // Only skip the water atom itself. Other atoms in the same unit can still be valid.
+                if (atomicUnit === unitW && hitLocalIdx === waterAtomIdx) continue;
+                if (isWater(atomicUnit, hitLocalIdx)) continue;
+
+                const hitFeat = unitsFeatures.get(atomicUnit.id);
+                if (!hitFeat || hitFeat.count === 0) continue;
+
+                const infoHit = Features.Info(structure, atomicUnit, hitFeat);
+
+                const { indices: fIdxs, offsets: fOff } = hitFeat.elementsIndex;
+                for (let k = fOff[hitLocalIdx], kl = fOff[hitLocalIdx + 1]; k < kl; k++) {
+                    const fi = fIdxs[k] as Features.FeatureIndex;
+                    const fType = hitFeat.types[fi];
+
+                    if (fType !== FeatureType.HydrogenDonor && fType !== FeatureType.HydrogenAcceptor) continue;
+
+                    const memberIdx = hitFeat.members[hitFeat.offsets[fi]] as StructureElement.UnitIndex;
+
+                    if (!props.backbone && isBackboneAtom(atomicUnit, memberIdx)) continue;
+
+                    atomicUnit.conformation.position(atomicUnit.elements[memberIdx], candidatePos);
+
+                    const distSq = Vec3.squaredDistance(candidatePos, wPos);
+                    if (distSq < legDistMinSq || distSq > legDistMaxSq) continue;
+
+                    infoHit.feature = fi;
+
+                    if (fType === FeatureType.HydrogenDonor) {
+                        const key = featureKey(atomicUnit.id, fi);
+                        if (donorKeys.has(key)) continue;
+
+                        if (checkGeometry(structure, infoHit, infoWAcc, legOpts)) {
+                            donorKeys.add(key);
+                            donors.push({
+                                unit: atomicUnit,
+                                featureIdx: fi,
+                                memberIdx,
+                                x: candidatePos[0],
+                                y: candidatePos[1],
+                                z: candidatePos[2],
+                                distSq,
+                            });
+                        }
+                    } else {
+                        const key = featureKey(atomicUnit.id, fi);
+                        if (acceptorKeys.has(key)) continue;
+
+                        if (checkGeometry(structure, infoWDon, infoHit, legOpts)) {
+                            acceptorKeys.add(key);
+                            acceptors.push({
+                                unit: atomicUnit,
+                                featureIdx: fi,
+                                memberIdx,
+                                x: candidatePos[0],
+                                y: candidatePos[1],
+                                z: candidatePos[2],
+                                distSq,
+                            });
+                        }
+                    }
+                }
+            }
+
+            for (const don of donors) {
+                for (const acc of acceptors) {
+                    // Reject bridges where donor and acceptor are the same physical atom
+                    // represented by different feature indices.
+                    if (don.unit === acc.unit && don.memberIdx === acc.memberIdx) continue;
+
+                    if (!checkOmega(don, wPos, acc, cosOmegaMin, cosOmegaMax)) continue;
+
+                    const combinedDistSq = don.distSq + acc.distSq;
+                    const donorKey = featureKey(don.unit.id, don.featureIdx);
+                    const acceptorKey = featureKey(acc.unit.id, acc.featureIdx);
+
+                    const existing = getBestBridge(best, donorKey, acceptorKey);
+                    if (!existing || combinedDistSq < existing.combinedDistSq) {
+                        setBestBridge(best, donorKey, acceptorKey, {
+                            contact: {
+                                unitA: don.unit.id,
+                                indexA: don.featureIdx,
+                                unitB: acc.unit.id,
+                                indexB: acc.featureIdx,
+                                unitM: unitW.id,
+                                indexMA: accFW,
+                                indexMB: donFW,
+                                props: { type: InteractionType.WaterBridge, flag: InteractionFlag.None },
+                            },
+                            combinedDistSq,
+                        });
+                    }
+                }
+            }
+        }
+    }
+
+    return bestBridgeValues(best).map(e => e.contact);
+}

src/mol-model-props/computed/representations/interactions-bridge-cylinder.ts

@@ -0,0 +1,400 @@
+/**
+ * Copyright (c) 2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ *
+ * @author Sebastian Bittrich <sebastian.m.bittrich@gmail.com>
+ */
+
+import { ParamDefinition as PD } from '../../../mol-util/param-definition';
+import { VisualContext } from '../../../mol-repr/visual';
+import { Structure, StructureElement, Unit } from '../../../mol-model/structure';
+import { Theme } from '../../../mol-theme/theme';
+import { Mesh } from '../../../mol-geo/geometry/mesh/mesh';
+import { Vec3 } from '../../../mol-math/linear-algebra';
+import { createLinkCylinderMesh, LinkCylinderParams, LinkStyle } from '../../../mol-repr/structure/visual/util/link';
+import { ComplexMeshParams, ComplexVisual, ComplexMeshVisual } from '../../../mol-repr/structure/complex-visual';
+import { VisualUpdateState } from '../../../mol-repr/util';
+import { PickingId } from '../../../mol-geo/geometry/picking';
+import { EmptyLoci, Loci } from '../../../mol-model/loci';
+import { NullLocation } from '../../../mol-model/location';
+import { Interval, OrderedSet } from '../../../mol-data/int';
+import { InteractionsProvider } from '../interactions';
+import { LocationIterator } from '../../../mol-geo/util/location-iterator';
+import { BridgeContacts, Bridges } from '../interactions/interactions';
+import { Sphere3D } from '../../../mol-math/geometry';
+import { InteractionsSharedParams } from './shared';
+import { Features } from '../interactions/features';
+
+type CanonicalLegIndices = {
+    endpointA: Int32Array
+    endpointB: Int32Array
+};
+
+const CanonicalLegIndicesCache = new WeakMap<BridgeContacts, CanonicalLegIndices>();
+
+function getCanonicalLegIndices(bridges: BridgeContacts): CanonicalLegIndices {
+    const cached = CanonicalLegIndicesCache.get(bridges);
+    if (cached) return cached;
+
+    const n = bridges.length;
+    const endpointA = new Int32Array(n);
+    const endpointB = new Int32Array(n);
+
+    const legA = new Map<string, number>();
+    const legB = new Map<string, number>();
+
+    for (let i = 0; i < n; i++) {
+        const b = bridges[i];
+
+        const kA = `${b.unitA}|${b.indexA}|${b.unitM}|${b.indexMA}`;
+        const kB = `${b.unitM}|${b.indexMB}|${b.unitB}|${b.indexB}`;
+
+        let ai = legA.get(kA);
+        if (ai === undefined) {
+            ai = i;
+            legA.set(kA, i);
+        }
+        endpointA[i] = ai;
+
+        let bi = legB.get(kB);
+        if (bi === undefined) {
+            bi = i;
+            legB.set(kB, i);
+        }
+        endpointB[i] = bi;
+    }
+
+    const indices = { endpointA, endpointB };
+    CanonicalLegIndicesCache.set(bridges, indices);
+    return indices;
+}
+
+function getFeatureMember(features: Features, featureIndex: Features.FeatureIndex): StructureElement.UnitIndex {
+    return features.members[features.offsets[featureIndex]] as StructureElement.UnitIndex;
+}
+
+function atomPosition(unit: Unit.Atomic, features: Features, featureIndex: Features.FeatureIndex, out: Vec3) {
+    const atomLocalIdx = getFeatureMember(features, featureIndex);
+    unit.conformation.position(unit.elements[atomLocalIdx], out);
+}
+
+function setFeatureLocation(
+    structure: Structure,
+    location: StructureElement.Location,
+    unitId: number,
+    features: Features,
+    featureIndex: Features.FeatureIndex
+) {
+    const unit = structure.unitMap.get(unitId) as Unit.Atomic;
+    const atomLocalIdx = getFeatureMember(features, featureIndex);
+
+    location.unit = unit;
+    location.element = unit.elements[atomLocalIdx];
+}
+
+function applyLegA(
+    bridgeIndex: number,
+    bridgeCount: number,
+    canonical: CanonicalLegIndices,
+    apply: (interval: Interval) => boolean
+) {
+    let changed = false;
+    const i = canonical.endpointA[bridgeIndex];
+
+    if (apply(Interval.ofSingleton(i))) changed = true;
+    if (apply(Interval.ofSingleton(i + bridgeCount))) changed = true;
+
+    return changed;
+}
+
+function applyLegB(
+    bridgeIndex: number,
+    bridgeCount: number,
+    canonical: CanonicalLegIndices,
+    apply: (interval: Interval) => boolean
+) {
+    let changed = false;
+    const i = canonical.endpointB[bridgeIndex];
+
+    if (apply(Interval.ofSingleton(i + 2 * bridgeCount))) changed = true;
+    if (apply(Interval.ofSingleton(i + 3 * bridgeCount))) changed = true;
+
+    return changed;
+}
+
+function createBridgeCylinderMesh(ctx: VisualContext, structure: Structure, theme: Theme, props: PD.Values<BridgeParams>, mesh?: Mesh) {
+    if (!structure.hasAtomic) return Mesh.createEmpty(mesh);
+
+    const interactions = InteractionsProvider.get(structure).value;
+    if (!interactions) return Mesh.createEmpty(mesh);
+
+    const { bridges, unitsFeatures } = interactions;
+
+    const n = bridges.length;
+    if (!n) return Mesh.createEmpty(mesh);
+
+    const l = StructureElement.Location.create(structure);
+    const { sizeFactor } = props;
+    const canonical = getCanonicalLegIndices(bridges);
+
+    const builderProps = {
+        // Four half-cylinders per bridge; createLinkCylinderMesh draws the A-side half per call:
+        //   [0,   n): A→mediator, forward  (A side)
+        //   [n,  2n): A→mediator, backward (mediator side)
+        //   [2n, 3n): mediator→B, forward  (mediator side)
+        //   [3n, 4n): mediator→B, backward (B side)
+        //
+        // When multiple bridges share the same physical leg, only the first
+        // occurrence is drawn; later ones map back to the canonical edge index.
+        linkCount: 4 * n,
+
+        position: (posA: Vec3, posB: Vec3, edgeIndex: number) => {
+            const b = bridges[edgeIndex % n];
+            const uM = structure.unitMap.get(b.unitM) as Unit.Atomic;
+            const fM = unitsFeatures.get(b.unitM);
+            const leg = Math.floor(edgeIndex / n);
+
+            if (leg === 0) {
+                const uA = structure.unitMap.get(b.unitA) as Unit.Atomic;
+                const fA = unitsFeatures.get(b.unitA);
+                atomPosition(uA, fA, b.indexA, posA);
+                atomPosition(uM, fM, b.indexMA, posB);
+            } else if (leg === 1) {
+                const uA = structure.unitMap.get(b.unitA) as Unit.Atomic;
+                const fA = unitsFeatures.get(b.unitA);
+                atomPosition(uM, fM, b.indexMA, posA);
+                atomPosition(uA, fA, b.indexA, posB);
+            } else if (leg === 2) {
+                const uB = structure.unitMap.get(b.unitB) as Unit.Atomic;
+                const fB = unitsFeatures.get(b.unitB);
+                atomPosition(uM, fM, b.indexMB, posA);
+                atomPosition(uB, fB, b.indexB, posB);
+            } else {
+                const uB = structure.unitMap.get(b.unitB) as Unit.Atomic;
+                const fB = unitsFeatures.get(b.unitB);
+                atomPosition(uB, fB, b.indexB, posA);
+                atomPosition(uM, fM, b.indexMB, posB);
+            }
+        },
+
+        ignore: (edgeIndex: number) => {
+            const bi = edgeIndex % n;
+            const leg = Math.floor(edgeIndex / n);
+
+            return leg <= 1
+                ? canonical.endpointA[bi] !== bi
+                : canonical.endpointB[bi] !== bi;
+        },
+
+        style: (_edgeIndex: number) => LinkStyle.Dashed,
+
+        radius: (edgeIndex: number) => {
+            const b = bridges[edgeIndex % n];
+            const leg = Math.floor(edgeIndex / n);
+            const isLegA = leg <= 1;
+
+            if (isLegA) {
+                const fA = unitsFeatures.get(b.unitA);
+                const fM = unitsFeatures.get(b.unitM);
+
+                setFeatureLocation(structure, l, b.unitA, fA, b.indexA);
+                const sizeA = theme.size.size(l);
+
+                setFeatureLocation(structure, l, b.unitM, fM, b.indexMA);
+                const sizeM = theme.size.size(l);
+
+                return Math.min(sizeA, sizeM) * sizeFactor;
+            } else {
+                const fM = unitsFeatures.get(b.unitM);
+                const fB = unitsFeatures.get(b.unitB);
+
+                setFeatureLocation(structure, l, b.unitM, fM, b.indexMB);
+                const sizeM = theme.size.size(l);
+
+                setFeatureLocation(structure, l, b.unitB, fB, b.indexB);
+                const sizeB = theme.size.size(l);
+
+                return Math.min(sizeM, sizeB) * sizeFactor;
+            }
+        },
+    };
+
+    const { mesh: m, boundingSphere } = createLinkCylinderMesh(ctx, builderProps, props, mesh);
+
+    if (boundingSphere) {
+        m.setBoundingSphere(boundingSphere);
+    } else if (m.triangleCount > 0) {
+        const sphere = Sphere3D.expand(Sphere3D(), structure.boundary.sphere, sizeFactor);
+        m.setBoundingSphere(sphere);
+    }
+
+    return m;
+}
+
+export const BridgeParams = {
+    ...ComplexMeshParams,
+    ...LinkCylinderParams,
+    ...InteractionsSharedParams,
+};
+export type BridgeParams = typeof BridgeParams
+
+export function BridgeVisual(materialId: number): ComplexVisual<BridgeParams> {
+    return ComplexMeshVisual<BridgeParams>({
+        defaultProps: PD.getDefaultValues(BridgeParams),
+        createGeometry: createBridgeCylinderMesh,
+        createLocationIterator: createBridgeIterator,
+        getLoci: getBridgeLoci,
+        eachLocation: eachBridgeInteraction,
+
+        setUpdateState: (
+            state: VisualUpdateState,
+            newProps: PD.Values<BridgeParams>,
+            currentProps: PD.Values<BridgeParams>,
+            newTheme: Theme,
+            currentTheme: Theme,
+            newStructure: Structure,
+            _currentStructure: Structure
+        ) => {
+            state.createGeometry = (
+                newProps.sizeFactor !== currentProps.sizeFactor ||
+                newProps.dashCount !== currentProps.dashCount ||
+                newProps.dashScale !== currentProps.dashScale ||
+                newProps.dashCap !== currentProps.dashCap ||
+                newProps.radialSegments !== currentProps.radialSegments ||
+                newTheme.size !== currentTheme.size
+            );
+
+            const interactionsHash = InteractionsProvider.get(newStructure).version;
+            if ((state.info.interactionsHash as number) !== interactionsHash) {
+                state.createGeometry = true;
+                state.updateTransform = true;
+                state.updateColor = true;
+                state.info.interactionsHash = interactionsHash;
+            }
+        }
+    }, materialId);
+}
+
+function getBridgeLoci(pickingId: PickingId, structure: Structure, id: number) {
+    const { objectId, groupId } = pickingId;
+    if (id !== objectId) return EmptyLoci;
+
+    const interactions = InteractionsProvider.get(structure).value;
+    if (!interactions) return EmptyLoci;
+
+    const { bridges, unitsFeatures } = interactions;
+    const n = bridges.length;
+
+    if (!n || groupId < 0 || groupId >= 4 * n) return EmptyLoci;
+
+    const bridgeIndex = groupId % n;
+
+    return Bridges.Loci({ structure, bridges, unitsFeatures }, [{ bridgeIndex }]);
+}
+
+const __unitMap = new Map<number, OrderedSet<StructureElement.UnitIndex>>();
+
+function eachBridgeInteraction(loci: Loci, structure: Structure, apply: (interval: Interval) => boolean, _isMarking: boolean) {
+    let changed = false;
+
+    if (Bridges.isLoci(loci)) {
+        if (!Structure.areEquivalent(loci.data.structure, structure)) return false;
+
+        const interactions = InteractionsProvider.get(structure).value;
+        if (!interactions) return false;
+
+        const { bridges } = interactions;
+        const n = bridges.length;
+        if (!n) return false;
+
+        const canonical = getCanonicalLegIndices(bridges);
+
+        for (const e of loci.elements) {
+            if (e.bridgeIndex < 0 || e.bridgeIndex >= n) continue;
+
+            if (applyLegA(e.bridgeIndex, n, canonical, apply)) changed = true;
+            if (applyLegB(e.bridgeIndex, n, canonical, apply)) changed = true;
+        }
+    } else if (StructureElement.Loci.is(loci)) {
+        if (!Structure.areEquivalent(loci.structure, structure)) return false;
+
+        const interactions = InteractionsProvider.get(structure).value;
+        if (!interactions) return false;
+
+        const { bridges, unitsFeatures } = interactions;
+        const n = bridges.length;
+        if (!n) return false;
+
+        const canonical = getCanonicalLegIndices(bridges);
+
+        __unitMap.clear();
+        for (const e of loci.elements) {
+            __unitMap.set(e.unit.id, e.indices);
+        }
+
+        for (let i = 0; i < n; i++) {
+            const b = bridges[i];
+
+            const indicesA = __unitMap.get(b.unitA);
+            const indicesM = __unitMap.get(b.unitM);
+            const indicesB = __unitMap.get(b.unitB);
+
+            if (!indicesA && !indicesM && !indicesB) continue;
+
+            let hitA = false;
+            if (indicesA) {
+                const fA = unitsFeatures.get(b.unitA);
+                const mi = getFeatureMember(fA, b.indexA);
+                hitA = OrderedSet.has(indicesA, mi);
+            }
+
+            let hitM = false;
+            if (indicesM) {
+                const fM = unitsFeatures.get(b.unitM);
+                const miA = getFeatureMember(fM, b.indexMA);
+                const miB = getFeatureMember(fM, b.indexMB);
+                hitM = OrderedSet.has(indicesM, miA) || OrderedSet.has(indicesM, miB);
+            }
+
+            let hitB = false;
+            if (indicesB) {
+                const fB = unitsFeatures.get(b.unitB);
+                const mi = getFeatureMember(fB, b.indexB);
+                hitB = OrderedSet.has(indicesB, mi);
+            }
+
+            if (hitA || hitM) {
+                if (applyLegA(i, n, canonical, apply)) changed = true;
+            }
+
+            if (hitB || hitM) {
+                if (applyLegB(i, n, canonical, apply)) changed = true;
+            }
+        }
+
+        __unitMap.clear();
+    }
+
+    return changed;
+}
+
+function createBridgeIterator(structure: Structure): LocationIterator {
+    const interactions = InteractionsProvider.get(structure).value;
+    if (!interactions) return LocationIterator(0, 1, 1, () => NullLocation, true);
+
+    const { bridges, unitsFeatures } = interactions;
+
+    const n = bridges.length;
+    const groupCount = 4 * n;
+    const instanceCount = 1;
+
+    const data: Bridges.Data = { structure, bridges, unitsFeatures };
+    const location = Bridges.Location(data);
+    const { element } = location;
+
+    const getLocation = (groupIndex: number) => {
+        element.bridgeIndex = n === 0 ? 0 : groupIndex % n;
+        return location;
+    };
+
+    return LocationIterator(groupCount, instanceCount, 1, getLocation, true);
+}

src/mol-model-props/computed/representations/interactions.ts

@@ -1,5 +1,5 @@
 /**
- * Copyright (c) 2019-2021 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2019-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  */
@@ -12,20 +12,23 @@ import { UnitsRepresentation, StructureRepresentation, StructureRepresentationSt
 import { InteractionsIntraUnitParams, InteractionsIntraUnitVisual } from './interactions-intra-unit-cylinder';
 import { InteractionsProvider } from '../interactions';
 import { InteractionsInterUnitParams, InteractionsInterUnitVisual } from './interactions-inter-unit-cylinder';
+import { BridgeParams, BridgeVisual } from './interactions-bridge-cylinder';
 import { CustomProperty } from '../../common/custom-property';
 import { getUnitKindsParam } from '../../../mol-repr/structure/params';
 
 const InteractionsVisuals = {
     'intra-unit': (ctx: RepresentationContext, getParams: RepresentationParamsGetter<Structure, InteractionsIntraUnitParams>) => UnitsRepresentation('Intra-unit interactions cylinder', ctx, getParams, InteractionsIntraUnitVisual),
     'inter-unit': (ctx: RepresentationContext, getParams: RepresentationParamsGetter<Structure, InteractionsInterUnitParams>) => ComplexRepresentation('Inter-unit interactions cylinder', ctx, getParams, InteractionsInterUnitVisual),
+    'bridge': (ctx: RepresentationContext, getParams: RepresentationParamsGetter<Structure, BridgeParams>) => ComplexRepresentation('Bridge cylinder', ctx, getParams, BridgeVisual),
 };
 
 export const InteractionsParams = {
     ...InteractionsIntraUnitParams,
     ...InteractionsInterUnitParams,
+    ...BridgeParams,
     unitKinds: getUnitKindsParam(['atomic']),
     sizeFactor: PD.Numeric(0.2, { min: 0.01, max: 1, step: 0.01 }),
-    visuals: PD.MultiSelect(['intra-unit', 'inter-unit'], PD.objectToOptions(InteractionsVisuals)),
+    visuals: PD.MultiSelect(['intra-unit', 'inter-unit', 'bridge'], PD.objectToOptions(InteractionsVisuals)),
 };
 export type InteractionsParams = typeof InteractionsParams
 export function getInteractionParams(ctx: ThemeRegistryContext, structure: Structure) {

src/mol-model-props/computed/themes/interaction-type.ts

@@ -1,7 +1,8 @@
 /**
- * Copyright (c) 2019-2020 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2019-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
+ * @author Sebastian Bittrich <sebastian.m.bittrich@gmail.com>
  */
 
 import { Location } from '../../../mol-model/location';
@@ -12,7 +13,7 @@ import { ThemeDataContext } from '../../../mol-theme/theme';
 import { ColorTheme, LocationColor } from '../../../mol-theme/color';
 import { InteractionType } from '../interactions/common';
 import { TableLegend } from '../../../mol-util/legend';
-import { Interactions } from '../interactions/interactions';
+import { Interactions, Bridges } from '../interactions/interactions';
 import { CustomProperty } from '../../common/custom-property';
 import { hash2 } from '../../../mol-data/util';
 import { ColorThemeCategory } from '../../../mol-theme/color/categories';
@@ -29,6 +30,7 @@ const InteractionTypeColors = ColorMap({
     CationPi: 0xFF8000,
     PiStacking: 0x8CB366,
     WeakHydrogenBond: 0xC5DDEC,
+    WaterBridge: 0x00CCEE,
 });
 
 const InteractionTypeColorTable: [string, Color][] = [
@@ -40,6 +42,7 @@ const InteractionTypeColorTable: [string, Color][] = [
     ['Cation Pi', InteractionTypeColors.CationPi],
     ['Pi Stacking', InteractionTypeColors.PiStacking],
     ['Weak HydrogenBond', InteractionTypeColors.WeakHydrogenBond],
+    ['Water Bridge', InteractionTypeColors.WaterBridge],
 ];
 
 function typeColor(type: InteractionType): Color {
@@ -60,6 +63,8 @@ function typeColor(type: InteractionType): Color {
             return InteractionTypeColors.PiStacking;
         case InteractionType.WeakHydrogenBond:
             return InteractionTypeColors.WeakHydrogenBond;
+        case InteractionType.WaterBridge:
+            return InteractionTypeColors.WaterBridge;
         case InteractionType.Unknown:
             return DefaultColor;
     }
@@ -91,6 +96,9 @@ export function InteractionTypeColorTheme(ctx: ThemeDataContext, props: PD.Value
                     return typeColor(contacts.edges[idx].props.type);
                 }
             }
+            if (Bridges.isLocation(location)) {
+                return typeColor(location.data.bridges[location.element.bridgeIndex].props.type);
+            }
             return DefaultColor;
         };
     } else {

src/mol-model/structure/structure/unit/rings.ts

@@ -1,5 +1,5 @@
 /**
- * Copyright (c) 2018-2025 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2018-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author David Sehnal <david.sehnal@gmail.com>
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
@@ -95,10 +95,22 @@ namespace UnitRing {
         Elements.SN, Elements.SB,
         Elements.BI
     ] as ElementSymbol[]);
+    /**
+     * Elements that are sp3 (and therefore non-aromatic) when degree >= 4 with no pi bonds.
+     * Excludes O (never realistically reaches degree 4) and N (quaternary N can be aromatic,
+     * but is guarded by the hasPiBond check below).
+     */
+    const Sp3RingCheckElements = new Set([
+        Elements.B, Elements.C, Elements.N,
+        Elements.SI, Elements.P, Elements.S,
+        Elements.GE, Elements.AS,
+        Elements.SN, Elements.SB,
+        Elements.BI
+    ] as ElementSymbol[]);
     const AromaticRingPlanarityThreshold = 0.05;
 
     export function isAromatic(unit: Unit.Atomic, ring: UnitRing): boolean {
-        const { elements, bonds: { b, offset, edgeProps: { flags } } } = unit;
+        const { elements, bonds: { b, offset, edgeProps: { flags, order } } } = unit;
         const { type_symbol, label_comp_id } = unit.model.atomicHierarchy.atoms;
 
         // ignore Proline (can be flat because of bad geometry)
@@ -120,6 +132,25 @@ namespace UnitRing {
                 }
             }
         }
+
+        for (let i = 0, il = ring.length; i < il; ++i) {
+            const aI = ring[i];
+            const elem = type_symbol.value(elements[aI]);
+            if (!Sp3RingCheckElements.has(elem)) continue;
+
+            let degree = 0;
+            let hasPiBond = false;
+            for (let j = offset[aI], jl = offset[aI + 1]; j < jl; ++j) {
+                degree += 1;
+                const f = flags[j];
+                const o = order[j];
+                if (BondType.is(BondType.Flag.Aromatic, f) || o === 2 || o === 3) {
+                    hasPiBond = true;
+                }
+            }
+            if (degree >= 4 && !hasPiBond) return false;
+        }
+
         if (aromaticBondCount === 2 * ring.length) return true;
         if (!hasAromaticRingElement) return false;
         if (ring.length < 5) return false;

src/mol-plugin-state/animation/built-in/camera-rock.ts

@@ -1,5 +1,5 @@
 /**
- * Copyright (c) 2022 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2022-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  */
@@ -12,7 +12,7 @@ import { degToRad } from '../../../mol-math/misc';
 import { ParamDefinition as PD } from '../../../mol-util/param-definition';
 import { PluginStateAnimation } from '../model';
 
-const _dir = Vec3(), _axis = Vec3(), _rot = Quat();
+const _dir = Vec3(), _axis = Vec3(), _rot = Quat(), _up = Vec3(), _side = Vec3();
 
 type State = { snapshot: Camera.Snapshot };
 
@@ -24,6 +24,7 @@ export const AnimateCameraRock = PluginStateAnimation.create({
         durationInMs: PD.Numeric(4000, { min: 100, max: 20000, step: 100 }),
         speed: PD.Numeric(1, { min: 1, max: 10, step: 1 }, { description: 'How many times to rock from side to side.' }),
         angle: PD.Numeric(10, { min: 0, max: 180, step: 1 }, { description: 'How many degrees to rotate in each direction.' }),
+        axis: PD.Vec3(Vec3.create(0, -1, 0), {}, { description: 'Axis of rotation in camera space' }),
     }),
     initialState: (p, ctx) => ({ snapshot: ctx.canvas3d!.camera.getSnapshot() }) as State,
     getDuration: p => ({ kind: 'fixed', durationMs: p.durationInMs }),
@@ -47,11 +48,25 @@ export const AnimateCameraRock = PluginStateAnimation.create({
         const angle = Math.sin(phase * ctx.params.speed * Math.PI * 2) * degToRad(ctx.params.angle);
 
         Vec3.sub(_dir, snapshot.position, snapshot.target);
-        Vec3.normalize(_axis, snapshot.up);
+
+        // Transform axis from camera space to world space
+        Vec3.normalize(_axis, _dir); // Z = view direction
+        Vec3.normalize(_up, snapshot.up); // Y = up
+        Vec3.cross(_side, _up, _axis); // X = right
+        Vec3.normalize(_side, _side);
+        const a = ctx.params.axis ?? Vec3.create(0, -1, 0); // default for backwards compatibility
+        Vec3.set(_axis,
+            a[0] * _side[0] + a[1] * _up[0] + a[2] * _axis[0],
+            a[0] * _side[1] + a[1] * _up[1] + a[2] * _axis[1],
+            a[0] * _side[2] + a[1] * _up[2] + a[2] * _axis[2]
+        );
+        Vec3.normalize(_axis, _axis);
+
         Quat.setAxisAngle(_rot, _axis, angle);
         Vec3.transformQuat(_dir, _dir, _rot);
+        Vec3.transformQuat(_up, snapshot.up, _rot);
         const position = Vec3.add(Vec3(), snapshot.target, _dir);
-        ctx.plugin.canvas3d?.requestCameraReset({ snapshot: { ...snapshot, position }, durationMs: 0 });
+        ctx.plugin.canvas3d?.requestCameraReset({ snapshot: { ...snapshot, position, up: _up }, durationMs: 0 });
 
         if (phase >= 0.99999) {
             return { kind: 'finished' };

src/mol-plugin-state/animation/built-in/camera-spin.ts

@@ -1,7 +1,8 @@
 /**
- * Copyright (c) 2020-2022 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2020-2026 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author David Sehnal <david.sehnal@gmail.com>
+ * @author Alexander Rose <alexander.rose@weirdbyte.de>
  */
 
 import { Camera } from '../../../mol-canvas3d/camera';
@@ -11,7 +12,7 @@ import { Vec3 } from '../../../mol-math/linear-algebra/3d/vec3';
 import { ParamDefinition as PD } from '../../../mol-util/param-definition';
 import { PluginStateAnimation } from '../model';
 
-const _dir = Vec3(), _axis = Vec3(), _rot = Quat();
+const _dir = Vec3(), _axis = Vec3(), _rot = Quat(), _up = Vec3(), _side = Vec3();
 
 type State = { snapshot: Camera.Snapshot };
 
@@ -22,7 +23,7 @@ export const AnimateCameraSpin = PluginStateAnimation.create({
     params: () => ({
         durationInMs: PD.Numeric(4000, { min: 100, max: 20000, step: 100 }),
         speed: PD.Numeric(1, { min: 1, max: 10, step: 1 }, { description: 'How many times to spin in the specified duration.' }),
-        direction: PD.Select<'cw' | 'ccw'>('cw', [['cw', 'Clockwise'], ['ccw', 'Counter Clockwise']], { cycle: true })
+        axis: PD.Vec3(Vec3.create(0, -1, 0), {}, { description: 'Axis of rotation in camera space' }),
     }),
     initialState: (_, ctx) => ({ snapshot: ctx.canvas3d?.camera.getSnapshot()! }) as State,
     getDuration: p => ({ kind: 'fixed', durationMs: p.durationInMs }),
@@ -42,14 +43,28 @@ export const AnimateCameraSpin = PluginStateAnimation.create({
         const phase = t.animation
             ? t.animation?.currentFrame / (t.animation.frameCount + 1)
             : clamp(t.current / ctx.params.durationInMs, 0, 1);
-        const angle = 2 * Math.PI * phase * ctx.params.speed * (ctx.params.direction === 'ccw' ? -1 : 1);
+        const angle = 2 * Math.PI * phase * ctx.params.speed;
 
         Vec3.sub(_dir, snapshot.position, snapshot.target);
-        Vec3.normalize(_axis, snapshot.up);
+
+        // Transform axis from camera space to world space
+        Vec3.normalize(_axis, _dir); // Z = view direction
+        Vec3.normalize(_up, snapshot.up); // Y = up
+        Vec3.cross(_side, _up, _axis); // X = right
+        Vec3.normalize(_side, _side);
+        const a = ctx.params.axis ?? Vec3.create(0, -1, 0); // default for backwards compatibility
+        Vec3.set(_axis,
+            a[0] * _side[0] + a[1] * _up[0] + a[2] * _axis[0],
+            a[0] * _side[1] + a[1] * _up[1] + a[2] * _axis[1],
+            a[0] * _side[2] + a[1] * _up[2] + a[2] * _axis[2]
+        );
+        Vec3.normalize(_axis, _axis);
+
         Quat.setAxisAngle(_rot, _axis, angle);
         Vec3.transformQuat(_dir, _dir, _rot);
+        Vec3.transformQuat(_up, snapshot.up, _rot);
         const position = Vec3.add(Vec3(), snapshot.target, _dir);
-        ctx.plugin.canvas3d?.requestCameraReset({ snapshot: { ...snapshot, position }, durationMs: 0 });
+        ctx.plugin.canvas3d?.requestCameraReset({ snapshot: { ...snapshot, position, up: _up }, durationMs: 0 });
 
         if (phase >= 0.99999) {
             return { kind: 'finished' };