Quasi-degenerate electronic wave functions

Improving the quality of the quasi-degenerate electronic wave functions and the description of bond breaking by the extended coupled-cluster theory... 

Let us consider a pair of the electronic wave functions, >i) and 2), tending to crossover at the nuclear coordinate Qo (Fig. 114). A proper electronic wave function A) should be considered as a (quasi) degenerate, and then a linear combination of the respective pair is appropriate ... 

The GMS wave function [1,2] combines the advantages of the MO and VB models, preserving the classical chemical structures, but dealing with self-consistently optimized orbitals. From a formal point of view, it is able to reproduce all VB or MO based variational electronic wave functions in its framework. Besides that, it can deal in a straightforward way with the nonadiabatic effects of degenerate or quasi-degenerate states, calculating their interaction and properties. 

Before closing this section, the extensions of the SAC/SAC-CI method to the quasi-degenerate and multi-reference cases are described. The first such proposal was made in 1985 as the MR-SAC method, in which the reference function was extended from the Hartee-Fock single-determinant to a general Cl type wave function [34]. Further, based on the consideration on the nature of electron correlations in quasidegenerate situations, a generalization of the exponential operator was made i.e. 

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