2-ELECTRON VALENCE INDEXES FROM THE KOHN-SHAM ORBITALS

The recent Hartree-Fock (HF) difference approach to the chemical valen ce indices (ionic and covalent), formulated in the framework of the pa ir-density matrix, is implemented within the Kohn-Sham (KS) density fu nctional theory (DFT). The valence numbers are quadratic in terms of d isplacements of the molecular spin-resolved charge-and-bond-order (CBO ) matrix elements, relative to values in the separated atoms limit (SA L). It is shown that the global valence represents a generalized ''dis tance'' quantity measuring a degree of similarity between the two CBO matrices: the molecular and SAL. Numerical values for typical molecule s exhibiting single and multiple bonds demonstrate that the KS orbital s give rise to these new bond valences in good agreement with both che mical and HF predictions. This KS bond multiplicity analysis is applie d to the chemisorption system including the allyl radical and a model surface cluster of molybdenum oxide. It is concluded that the quadrati c valence analysis represents a valuable procedure for extracting usef ul chemical information from standard DFT calculations. (C) 1997 John Wiley & Sons, Inc.