Davidson‐type corrections for quasidegenerate states

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Abstract
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.