Class IV charge models: A new semiempirical approach in quantum chemistry

Abstract

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 rms errors of 0.30 and 0.26 D, respectively, in the dipole moments of a set of 195 neutral molecules consisting of 103 molecules containing H, C, N and O, covering variations of multiple common organic functional groups, 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.

Contribution of a Solute's Chiral Solvent Imprint to Optical Rotation

“…[2] The OPLS-AA force field [3] with flexible solute and solvent geometries were used in the simulations. AM1 [4] [5] partial atomic charges scaled by 1.14 for the solute and solvent were used in the MC simulations. A more detailed description of the MC move and the atomic charge partitioning scheme can be found in [1].…”

Charge Model 4 and Intramolecular Charge Polarization

“…As in previous CMx models, 7,8,[23][24][25][26] the charges for the CM4 model are mapped from Class II charges obtained using population analysis by the following formula:…”

“…Second-generation electrostatic fitting algorithms such as RESP 20 include restraints to tame unphysical conformational dependences that sometimes occur 21,22 in electrostatic fitting. Finally, Class IV charges 8 are defined as charges that accurately reproduce or predict either charge-dependent experimental observables or well defined observables obtained by well converged quantum mechanical calculations.…”

“…A series of Class IV charge models 7,8,[23][24][25][26] has been developed for molecular orbital theory and density functional theory (DFT), including ab initio Hartree-Fock (HF) theory and hybrid DFT as special cases. These development efforts led to the recently proposed Charge Model 4 (CM4).…”

“…Using a simple functional form for the mapping, the CM4 model provides an accurate, efficient, and stable means of assigning partial atomic charges. The CM1 charge model 8 was developed only for neglect-of-diatomic-differentialoverlap theory, but CM2, [23][24][25] CM3, 26 and CM4 7 may be used with ab initio HF theory and DFT. In this article, we extended the CM4 model so that it can be used with any basis set from for which we previously parameterized a CMx model (x = 2, 3, or 4).…”