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

The concept of hybridization displacement charge (HDC) applied earlier with in 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 electr ostatic potentials (MEP) near the various atoms on the corresponding van de r Waals surfaces were calculated. Molecular dipole moments and MEP values o btained using HDC combined with Lowdin charges were compared with those obt ained 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 confi gurations (CIS). There is a good linear correlation between the following q uantities: (a) dipole moments obtained using HDC combined with Lowdin charg es 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 Lowdin charges and MEP minima obtained using RHF/6-31G** wave functions. Th e displacement directions of HDC associated with multiply bonded atoms are found to depend strongly on molecular shape. (C) 2001 Elsevier Science B.V. All rights reserved.