Atomic charges from atomic polar tensors: A comparison of methods

Different definitions of atomic charge obtained by DFT computed atomic polar tensors are compared in the case of fluorine and chlorine substituted ethylenes. Based on the analytic expressions of atomic polar tensors (APTs) in terms of infrared equilibrium charges and charge fluxes (ECCF) it is shown that the GAPT charges, routinely provided with quantum chemical predictions of vibrational spectra, include a large contribution of charge fluxes that could make them unreliable for a quantitative investigation of molecular charge distribution at equilibrium and poorly transferable from one molecule to molecules sharing the same chemical groups. On the other hand, within the ECCF model, IR equilibrium charges are free from spurious contributions due to dynamical charge rearrangement and they give a description in very good agreement with other popular population schemes based on the fitting of the molecular electrostatic potentials. Furthermore, IR charges obtained from calculated APTs by DFT method give a good quantitative description of the intramolecular chemical/physical effects which take place in molecules, in agreement with the picture provided by ECCF parametrization of experimental IR intensities.