MODIFIED VALENCE INDEXES FROM THE 2-PARTICLE DENSITY-MATRIX

The modified ionic and covalent valence indices are introduced, define d in the framework of the two-particle density matrix, with respect to the reference state of separated atoms or ions (SAL). They include on ly quadratic contributions in changes of the molecular charge-and-bond order matrix elements, relative to the SAL. General properties of the modified valence indices am examined and illustrative qualitative res ults for model systems are presented. Numerical UHF SCF MG valence dat a for selected diatomic and triatomic molecules are reported and inter preted in terms of the valence saturation effect and the ionic vs. cov alent valence competition. A three-orbital valence model of a symmetri c transition state of the bond-forming-bond-breaking reaction supports the BEBO model postulate of preservation of the total ''bond order.'' The model predictions are compared with the UHF numerical values. (C) 1994 John Wiley & Sons, Inc.