Multi-reference averaged quadratic coupled-cluster method: a size-extensive modification of multi-reference CI

Abstract An extension of the earlier multi-reference linearized coupled-cluster method to include quadratic EPV terms in an averaged way is presented. The resulting functional is conceptually similar to the averaged coupled pair functional but it offers superior performance particularly with small reference spaces. This is demonstrated on the ozone molecule using a two electron-two orbital GVB-type reference function.

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