Class IV Charge Models: A New Semiempirical Approach in Quantum Chemistry

Storer, J. W.; Giesen, D. J.; Cramer, C. J.; Truhlar, D. G.
J. Comput.-Aid. Mol. Des. 1995, 9, 87.

We propose a new criterion for defining partial charges on atoms in molecules, namely that physical observables calculated from those partial charges should be as accurate as possible. We also propose a method to obtain such charges based on a mapping from approximate electronic wave functions. The method is illustrated by parameterizing two new charge models called AM1-CM1A and PM3-CM1P, based on experimental dipole moments and, respectively, on AM1 and PM3 semiempirical electronic wave functions. These charge models yield root-mean-square errors of 0.30 and 0.26 Debyes respectively in the dipole moments of 195 neutral molecules consisting of 103 molecules containing H, C, N, and O covering variations of multiple common organic functional groups, and 68 fluorides, chlorides, bromides, and iodides, 15 compounds containing H, C, Si or S, and 9 compounds containing C-S-O or C-N-O linkages. In addition, partial charges computed with this method agree extremely well with high level ab initio calculations for both neutral compounds and ions. The CM1 charge models provide a more accurate point charge representation of the dipole moment than provided by most previously available partial charges, and they are far less expensive to compute.

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