Davidson-type corrections for quasidegenerate states

Two new extensions of Davidson-type size-consistency collections to the case of quasidegenerate states are presented and applied to a simple model system in which the degree of quasidegeneracy can be continuously varied. The eight-electron model studied is a minimum basis set model involving eight hydrogen atoms in various spatial arrangements. The results are compared with the full CI energies as well as with those obtained with the CI-D, MRCI-D, coupled-pair, and linear coupled-pair methods. Two extensions of the Davidson correction recently devised by Paldus are also employed. The results indicate that all the Davidson-type corrections yield reliable correlation energies in the case of strong quasidegeneracies.

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