THE DESCRIPTION OF EXCITED ELECTRONIC STATES BY CONFIGURATION INTERACTION THEORY

Abstract

The theoretical description of excited electronic states by ab initio SCF and CI techniques has been shown to be quite satisfactory for $^{1.3}$(n $\longrightarrow \pi^{4}$), $^{3}$($\pi\longrightarrow \pi^{4}$) and certain Rydberg states in a variety of molecules. However, the corresponding description of $^{3}$($\pi\longrightarrow \pi^{4}$) states frequently is found to be significantly is error. Possible deficiencies in different types of theoretical treatments have been attributed to inadequate basis sets or to incomplete Cl calculations, the latter being due to a greater than expected role of $\sigma\rightarrow\sigma^{4}$ excitations in mixing with the primary ($\pi\longrightarrow \pi^{4})$ excitation. The objective of the present work is to develop a procedure for obtaining the optimum $\sigma^{4}$ orbitals by maximization of the interaction with the primary configurations of the states of interest. Two procedures are discussed. The first involves the formulation of configurations $\Phi_{\sigma\rightarrow\sigma^{*}}$ by single excitation from the ground state where the $\sigma^{*}$ orbital is determined by maximization of the interaction with the primary configurations of the excited state. The second procedure is based on an analysis of excited state wave-functions in terms of natural orbitals, followed by single excitations, $\sigma\rightarrow\sigma^{*}$ from individual natural orbital components. Results of ab initio CI treatments of the ground and excited electronic states of ethylene, formaldehyde, diimide and pyrazine are discussed.