THE ORTHOGONAL VALENCE-BOND INTERPRETATION OF AB-INITIO CHEMISORPTION CLUSTER MODEL WAVE-FUNCTIONS

Ab initio cluster model self consistent field (SCF) and complete active space self consistent field (CASSCF) wavefunctions have been obtained for some representative chemisorption systems. These wavefunctions have been analyzed in terms of orthogonal valence bond, (OVB) components. The OVB analysis permits to understand the main features of non-dynamical correlation in the chemisorption bond. A simple effective Hamiltonian-based approach has been used to account for dynamical correlation. For essentially covalent bonds it has been found that electronic correlation decreases the contribution of ionic forms. However, the ionic contribution is increased by non-dynamical correlation for 0 on Cu (100), which may be described as an ionic bond. A discussion about the validity limits of the OVB analysis is also included.