IMERGE-FEP: Improving Relative Free Energy Calculation Convergence with Chemical Intermediates

Alchemical free energy calculations are becoming an increasingly prevalent tool in drug discovery efforts. Over the past decade, significant progress has been made in automating various aspects of this technique. However, one aspect hampering wider application is the construction of perturbation networks to connect ligands of interest. More specifically, ligand pairs with large dissimilarities should be avoided since they can lower convergence and decrease accuracy. Here, we propose a technique for automatic generation of intermediate molecules to break up problematic edges─calculations connecting two different ligands or molecules─into smaller perturbations. To this end, a modular tool was developed that generates intermediates for a molecule pair by enumerating R-group combinations called IMERGE-FEP (Intermediate MolEculaR GEnerator for Free Energy Perturbation). Intermediate enumeration of multiple, representative congeneric series showed that intermediates increase similarity regarding shared substructures, geometry, and LOMAP scores. Taken together, this tool eases integration of intermediate steps into free energy calculation protocols.