More reliable partial atomic charges when using diffuse basis sets

We present a method that alleviates some of the sensitivity to the inclusion of diffuse basis functions when calculating partial atomic charges from a Löwdin population analysis. This new method locally redistributes that part of the Löwdin population that comes from diffuse basis functions so that the final charges closely resemble those calculated without diffuse functions. We call this method the redistributed Löwdin population analysis (RLPA). The method contains one parameter for each atomic number, and we optimized the parameter for the 6-31+G(d) basis set. The method has been tested on compounds that contain H, Li, C, N, O, F, Si, P, S, Cl, and Br. For a test set of 398 compounds with experimental and high-level theoretical dipole moments, the dipole moments derived from the charges obtained by standard Löwdin population analysis have errors 35% larger than those obtained by the corresponding RLPA using the same basis set. In judging the quality of the RLPA with respect to the test set of dipole moments, we have also found that dipole moments derived from Mulliken population analysis have errors 120% larger than those derived from RLPA for the same basis set. The new method is particularly successful for the 207 systems containing only first row atoms (H, C, N, O, F) for which the errors in the dipole moments computed from the partial atomic charges obtained by standard Löwdin and Mulliken analysis are respectively 115 and 419% larger than those obtained by RLPA.