EXPLORING BONDING PATTERNS OF MOLECULAR-SYSTEMS USING QUANTUM-MECHANICAL BOND MULTIPLICITIES

One of the fundamental chemical concepts, the structural formula of a chemical species, which visualizes bonding patterns in a given molecul ar system, can also be given a more flexible and quantified form in te rms of alternative quantum-mechanical measures of bond-orders. The rec ently developed difference approach, based upon the one-determinantal wavefunction [Hartree Fock (HF) and Kohn-Sham (KS) theories], compares the charge-and-bond-order matrix in a molecule (P) and that in the se parated atoms limit (P-0, SAL) defined in the orthogonal basis set of atomic functions. In this approach the atomic and diatomic contributio ns to the molecularly averaged difference between these matrices, [Del ta P]/2 = 1/2tr[P(P-0 - P)], are used to define bond multiplicities. T he main purpose of this work is to examine the overall performance of such KS and HF bonding indices, when applied to the set of chemically well defined reference systems, and to use them to investigate bond-or ders in more challenging molecules, which have recently been the subje ct of independent studies. Manifestations of changing bond ionicity by these effective bond-orders are examined for a series of diatomics. S ome methodological aspects associated with alternative SAL choices are discussed and tested.