Many molecular phenomena can only be properly described using multiconfiguration wavefunctions, e.g., bond breaking/making, diradicals, many excited states, and chemical reactions. A common approach is to use a multiconfiguration wavefunction that includes all possible configurations constructed from a set of “active” orbitals: CASSCF or FORS. Although these wavefunctions provide an excellent zero-order description of molecular phenomena, the number of configurations in these wavefunctions increases exponentially with the number of active electrons and orbitals. In addition, the essential features of the electronic structure of the molecule are hidden in the delocalized, orthogonal orbitals and massive set of configurations. Spin-coupled generalized valence bond (SCGVB) wavefunctions can also be used to describe the multiconfiguration character of molecules. The SCGVB wavefunction provides a compelling orbital description of a broad range of molecular phenomena and accounts for most, if not all of the non-dynamical correlation. Although the non-orthogonality of the SCGVB orbitals complicates the calculation of dynamical correlation effects, this problem can be addressed by (1) recasting the SCGVB wavefunction in terms of its natural orbitals and (2) using the resulting SCGVB(NO) wavefunction as the zero-order wavefunction in a conventional multireference configuration interaction calculation. In this presentation we will describe SCGVB theory and its use in multireference configuration interaction calculations.

SCGVB-based multiconfiguration description of molecules

2019 Fall National Meeting

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