APPROXIMATE COUPLED-CLUSTER METHODS EMPLOYING SPLIT CLUSTER AMPLITUDES - IMPLEMENTATION OF AN ALMOST-LINEAR COUPLED-CLUSTER FORMALISM

A new approximation strategy, split-amplitude strategy, useful within the framework of the coupled-cluster (CC). methodology is proposed. It consists in representing the individual cluster amplitudes as a sum o f two components, one of fixed value, which may be obtained from exter nal sources, and the other determined from a set of modified CC equati ons. This approach provides new possibilities of absorbing information concerning the values of cluster amplitudes from independent calculat ions. By properly choosing the fixed amplitude components, one may sub stantially reduce the magnitudes of the most significant amplitudes to be determined for the state considered, which in turn causes that the known approximation procedures are more justifiable when applied to t he modified CC equations than to the equations of the standard CC appr oaches. The split-amplitude strategy has been employed to setting up s everal almost-linear CC (AL-CC) approaches of a single reference type corresponding to the basic CC methods. These low-cost approaches seem to be useful. in theories of processes involving nondynamically correl ated (quasidegenerate) states. The AL-CC methods have been applied to the ground states for various geometries of the following molecular sy stems: H8, H2O, BH, and HF. It is found that the energies obtained for a given AL-CC approach are very close to those for the corresponding basic CC method, which is,true even for strongly quasidegenerate state s. (C) 1998 American Institute of Physics. [S0021-9606(98)02039-X].