Charge Model 3:  A Class IV Charge Model Based on Hybrid Density Functional Theory with Variable Exchange

We present a new class IV charge model. The model, called Charge Model 3 (CM3), is designed to be able to obtain accurate partial charges from hybrid density functional calculations with a variable amount of Hartree−Fock exchange and with or without diffuse functions in the basis. The model maps atomic partial charges obtained by Löwdin or redistributed Löwdin population analysis into improved (class IV) charges that reproduce accurate charge-dependent observables for molecules containing H, Li, C, N, O, F, Si, S, P, Cl, and Br. The hybrid density functional theory we use here is based on Adamo and Barone's modified Perdew−Wang (mPW) gradient-corrected exchange functional and the PW91 gradient corrected correlation functional. These parametrizations can be used with any arbitrary fraction of Hartree−Fock exchange in conjunction with any of the five basis sets, MIDI!, MIDI!6D, 6-31G*, 6-31+G*, and 6-31+G**. We also present two parametrizations for Hartree−Fock theory employing the MIDI!6D and 6-31G* basis sets. Dipole moments computed from CM3 atomic point charges have root-mean-square errors between 0.26 and 0.40 D and mean unsigned errors in the range 0.19−0.28 D compared to experiment.