The Qualitative Rationalization and Prediction of “Correlation Effects” in “Complex” Ground State Molecules

Abstract

One of the great advantages of MOVB theory is that it forms the basis for a logical and coherent interpretation of chemical phenomena. The situation is quite different in the case of MO theory: Within this framework, one typically solves the problem at the SCF-MO level and then corrects the solution by re-solving it at the SCF-MO-CI level. In the process, one generates two apparently distinct conceptual frameworks with the result that the appearance is created that there are “MO effects” and “CI effects”. This is quite inappropriate since the “CI effects” are nothing else but consequences of the solution of the SCF-MO equations, unless by “CI effects” one implies the chemical consequences of nonvalence orbitals which are not included in the monodeterminantal calculation. Perusal of the vast theoretical literature reveals that this point has not been properly appreciated. We believe that this is due to the fact that there has been no conceptual tool capable of revealing the nature of error involved at the Single Determinant (SD) MO level and how it is linked to fundamental electronic mechanisms which are grossly reproduced by SD MO theory. With MOVB theory as our weapon, we now attempt to answer the following question: What is the meaning of the term, “valence correlation effect”, or, more briefly, “correlation effect”?