A Class IV Charge Model for Boron Based on Hybrid Density Functional Theory

We present a class IV charge model, in particular charge model 3 (CM3), for molecules containing boron. The model is designed to be able to obtain particularly useful partial atomic charges by mapping (class II) partial atomic charges obtained by Löwdin population analysis into improved (class IV) charges that reproduce accurate charge-dependent observables. To train the model, we mainly use dipole moments as the observables, and we have developed a training set of 43 accurate dipole moments and one quadrupole moment for molecules containing B in addition to H, C, N, O, and/or F. In the present paper we report CM3 parameters for use with hybrid density functional theory, in particular with Adamo and Barone's modified Perdew−Wang (mPW) gradient-corrected exchange functional, the PW91 gradient-corrected correlation functional, 25% Hartree−Fock exchange, and the popular 6-31G* basis set. Dipole moments of boron-containing molecules computed from CM3 atomic point charges have root-mean-square errors of only 0.13 D and mean unsigned errors of 0.10 D as compared to experiment or high level of theory.