Charges derived from electrostatic potentials: Exploring dependence on theory and geometry optimization levels for dipole moments

Three procedures based on the charges derived by fitting the ESP were examined, the charge from electrostatic potential (CHELP), charge from ESP grid based (CHELPG) by Breneman and Wiberg and the method of Merz and Kollman (MK). The ability of these charges to reproduce SCF and experimental dipole moments was evaluated for 89 molecules among hydrocarbons, alkyl halides, aromatics, oxygen and nitrogen compounds. It is shown that accuracy for dipole moments depends more on the theory level and geometry optimization than on the charge models used. From a computational viewpoint, the DFT methods with AM1 geometry optimization provide the best accuracy/cost ratio for the calculation of molecular dipole moments with respect to experimental data, with mean absolute deviation smaller than 0.19 D for total database. All three methods reproduce well SCF dipole moments, but charges derived from CHELP fit have deviations as much as twice than CHELPG and MK methods.