Approximate coupled cluster methods based on a split-amplitude strategy

A simple strategy for approximating the sets of nonlinear equations underlying 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 equations. This strategy provides new possibilities of absorbing information concerning the values of cluster amplitudes obtained from independent sources. This strategy has been employed by setting up several almost-linear CC (AL-CC) approaches of single-reference type for the description of quasidegenerate states. Pilot calculations performed for the H4 model indicate that these low-cost approaches yield accurate results even in the strong quasidegenerate regime.