Models and Methods in Theoretical & Computational Chemistry
Thom H. Dunning, Jr. and L. T. Xu
Department of Chemistry, University of Washington, Seattle, Washington
Theoretical models and computational methods are both critical to the advancement of chemical theory. Theoretical models enable the systematization of chemical knowledge, while computational methods enable the prediction of molecular properties. The most common theoretical model of the electronic structure of molecules is molecular orbital theory.
However, many molecules and molecular phenomena can only be described using multi-configuration wave functions. The valence CASSCF wave function provides a powerful multi-configuration description of the electronic structure of molecules. However, it does not provide an interpretable theoretical model and rapidly becomes computationally intractable as the number of active electrons and orbitals increases.
We are exploring the use of spin-coupled generalized valence bond (SCGVB) theory to address the multi configuration character of molecules. The SCGVB wave function, which accounts for non-dynamical correlation effects, provides a compelling orbital description of a broad range of molecular phenomena. Although the SCGVB orbitals are non-orthogonal, the SCGVB wave function can be recast in terms of a set of orthonormal natural orbitals,
enabling multi-reference configuration interaction calculations to be performed to capture the effects of dynamical correlation. We will describe SCGVB theory and its use in multi-reference configuration interaction calculations. Along the way we will highlight the connection of the this work to the research of Professor Ruedenberg and his colleagues at Iowa State University