Derivation of class II force fields. VIII. Derivation of a general quantummechanical force field for organic compounds

A class II valence force field covering a broad range of organic molecules has been derived employing ab initio quantum mechanical "observables." The procedure includes selecting representative molecules and molecular structu res, and systematically sampling their energy surfaces as described by ener gies and energy first and second derivatives with respect to molecular defo rmations. In this article the procedure for fitting the force field paramet ers to these energies and energy derivatives is briefly reviewed. The appli cation of the methodology to the derivation of a class II quantum mechanica l force field (QMFF) for 32 organic functional groups is then described. A training set of 400 molecules spanning the 32 functional groups was used to parameterize the force field. The molecular families comprising the functi onal groups and, within each family, the torsional angles used to sample di fferent conformers, are described. The number of stationary points (equilib ria and transition states) for these molecules is given for each functional group. This set contains 1324 stationary structures, with 718 minimum ener gy structures and 606 transition states. The quality of the fit to the quan tum data is gauged based on the deviations between the ab initio and force field energies and energy derivatives. The accuracy with which the QMFF rep roduces the ab initio molecular bond lengths, bond angles, torsional angles , vibrational frequencies, and conformational energies is then given for ea ch functional group. Consistently good accuracy is found for these computed properties for the various types of molecules. This demonstrates that the methodology is broadly applicable for the derivation of force field paramet ers across widely differing types of molecular structures. (C) 2001 John Wi ley & Sons, Inc.