MULTIREFERENCE AVERAGED QUADRATIC COUPLED-CLUSTER METHOD - A SIZE-EXTENSIVE MODIFICATION OF MULTIREFERENCE CI

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 cou pled pair functional but it offers superior performance particularly w ith small reference spaces. This is demonstrated on the ozone molecule using a two electron-two orbital GVB-type reference function.