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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.

1The average local electrostatic potential function, V ( r ) / p (~) , is calculated for 87 atoms, Li -Ac, in the ground state using the nonrelativistic average-over-configuration numerical Hartree-Fock density. It is found empirically that in a given atom the shell boundaries are expressed as the successively increasing maxima in V(r)/p(r) and the outermost maximum presents good approximate estimates of the core-valence separation in atoms. The likeness in behavior of V ( r > / p ( r ) at each shell boundary with the maximum hardness principle is discussed. The single-exponent-fit parameters for the electron density in the valency region are provided for all atoms. 0

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