A non-empirical electronegativity equalization scheme. Theory and applications using isolated atom properties

The electronegativity equalization principle, originally formulated by Sanderson and proven quantum mechanically by Parr et al., within the framework of density functional theory, is studied numerically. A first order expression for the electronegativity x of an atom A in a molecule is derived, based on the change, upon molecule formation, of the number of electrons NA and the external potential vA(r) of this atom with respect to the isolated atom case. The relative importance of the two corrections to the isolated atom electronegativity is discussed. Furthermore, the expression was identified with other density functional theory based electronegativity equalization formalisms and was applied to small molecular systems: the resulting charge distributions and molecular electronegativities showed a fair correlation with other estimations for these quantities. The importance of second order terms is also discussed, together with an extension, permitting the use of functional group electronegativities and hardnesses.