Dependence of the hardness of atoms in molecules on the local environment: An ab initio study

The effect of the deformation of the electron cloud on the chemical hardness of atoms in molecules was studied by ab initio calculations at the CCSD(T)/6-31++G(d,p) level. The influence of the molecular environment generally results in an increased hardness of the atoms. The extent of this effect depends on the kind of coordination as well as on the bond distances. Higher coordination results in higher atomic hardnesses; a shortening of the bond distances results in an exponential increase of this parameter. The hardening coefficient of an atom in a molecule as a function of the distance seems to obey an exponentional decaying regime, of which the falloff parameter correlates with the atomic polarizability. The present approach allows to improve existing electronegativity equalization schemes by explicitly taking into account the dependence of the hardness parameter on the molecular geometry.