An adaptive coupled-cluster theory: @CC approach

A formulation of an adaptive coupled-cluster theory is presented. The method automatically "adjusts" to any state of an electronic system and converges to the full CI limit, thus being capable of describing both single- and multireference phenomena. Adaptivity is accomplished through a guided selection of a compact set of cluster amplitudes as required for a proper description of the electronic system under consideration. The approach suggested is of "black-box" type. A special importance-selection function (discriminatory function) is explicitly introduced for the guided selection of variables involved in the theoretical model. The method is tested on molecules which exhibit strong multireference character in the region of chemical bond elongation. An unambiguous comparison with formally exact full CI solutions shows that the method is capable of providing mHartee accuracy using a rather compact set of cluster amplitudes.