Class IV charge model for the self-consistent charge density-functional tight-binding method

Class IV charges obtained using charge model 3 (CM3) have been shown to provide a realistic description of molecular charge distributions, even when obtained by mapping from highly approximate semiempirical wave functions. In the present study, the CM3 approach is extended to the self-consistent charge density-functional tight-binding (SCC-DFTB) method. Before mapping, the mean-signed error in 219 electric dipole moments obtained by Mulliken analysis is -0.46 D, and the root-mean-square error is 0.72 D. After CM3 mapping, these errors are decreased to -0.001 and 0.31 D, respectively. The resulting charge model, denoted CM3/SCC-DFTB, should be very useful (i) for obtaining reliable charges for large molecules, nanostructures, and macromolecular systems and (ii) for representing solute charge distributions when computing the electrostatic potential or the electrostatic contribution to solvation free energies.