Ways to Improve Expectation Values

The method is based on the following procedure.33 All possible doubly excited configurations are generated from the Hartree-Fock function and their contributions to the second-order Rayleigh-Schrodinger perturbation theory energy computed. Approximately 100 of the most important are used for a Cl calculation, all singly 

Methods in category (a) represent an overall improvement in the wavefunctlon and often require a major computational effort. There is an improvement in the energy and possibly in the expectation values of other operators, but at present the difficulties make them generally unattractive for calculations on medium or large molecules or for studies on smallish molecules at several different geometries. The second category contains methods which do not introduce correlation into the wave-function rather they attempt to tailor wavefunctions for specific purposes. 

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