Study of hybridization displacement charge distribution using ab initio wave functions and evaluation of molecular electrostatic properties

Abstract

The concept of hybridization displacement charge (HDC) applied earlier within the framework of semi-empirical methods has now been extended to the ab initio level. The magnitudes of HDC as well as their displacements from the corresponding nuclear sites, molecular dipole moments and molecular electrostatic potentials (MEP) near the various atoms on the corresponding van der Waals surfaces were calculated. Molecular dipole moments and MEP values obtained using HDC combined with Löwdin charges were compared with those obtained using the MEP-derived CHelpG charges. The calculations were performed on several molecules in the ground state at the SCF level using the STO-6G and 6-31G∗∗ basis sets. In a few cases, excited state properties were also studied employing configuration interaction involving singly excited configurations (CIS). There is a good linear correlation between the following quantities: (a) dipole moments obtained using HDC combined with Löwdin charges and CHelpG charges, (b) surface MEP values obtained by the two types of point charges, and (c) surface MEP values obtained using HDC combined with Löwdin charges and MEP minima obtained using RHF/6-31G∗∗ wave functions. The displacement directions of HDC associated with multiply bonded atoms are found to depend strongly on molecular shape.