APPROXIMATE COUPLED-CLUSTER METHODS BASED ON A SPLIT-AMPLITUDE STRATEGY

A simple strategy for approximating the sets of nonlinear equations un derlying the coupled cluster (CC) methods is proposed. It consists of splitting the individual cluster amplitudes into two components - one of fixed value and the other determined from a set of modified CC equa tions. This strategy provides new possibilities of absorbing informati on concerning the values of cluster amplitudes obtained from independe nt sources. This strategy has been employed by setting up several almo st-linear CC (AL-CC) approaches of single-reference type for the descr iption of quasidegenerate states. Pilot calculations performed for the H4 model indicate that these low-cost approaches yield accurate resul ts even in the strong quasidegenerate regime.