Charge flux and electrostatic forces in planar molecules

The electrostatic forces exerted by external point charges on model compounds are calculated ab initio and analyzed. It is shown that in general molecules cannot be represented as collections of isolated point charges interacting through vacuum. The distortion of the molecular geometry, caused by the external source, leads to an intramolecular charge redistribution ("charge flux") that modifies the static pairwise force, in some cases drastically. In general, charge flux introduces a dynamical anisotropy into electrostatic forces that is usually much larger than the static anisotropy due to atomic dipoles and higher atomic multipoles. The effect of charge flux is also larger than that of polarizability. Because of charge flux, electrostatic forces on nuclei in molecules are, in general, nonlocal and nonpairwise. Rather, electrostatic forces on nuclei in molecules are determined by the response of all other nuclear sites to the external source, and particularly the sites that are closest to the perturbing source. Consequently, flux forces may oppose the forces from the static multipoles and even override them. The calculation of charge flux parameters is not straightforward with common methods of determining atomic charges. This problem is solved by using the recently introduced force related (FR) atomic multipoles which are well-defined derivatives of quantum mechanical expectation values. The FR atomic multipoles and their flux successfully reproduce the ab initio forces in the molecules discussed in this work.