RDKit-based implementation of the method described in:

Estimation of Synthetic Accessibility Score of Drug-like Molecules based on Molecular Complexity and Fragment Contributions
Peter Ertl and Ansgar Schuffenhauer
Journal of Cheminformatics 1:8 (2009)
http://www.jcheminf.com/content/1/1/8

Contribution from Peter Ertl and Greg Landrum