Multi-determinant wave functions electron correlation methods

HF theory only accounts for the average electron-electron interactions, and consequently neglects the correlation between electrons. Methods that include electron correlation require a multi-determinant wave function, since HF is the best singledeterminant wave function. Multi-determinant methods are computationally much more involved than the HF model, but can generate results that systematically approach the exact solution of the Schrodinger equation. These methods are described in Chapter 4. 

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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