MODIFIED COUPLED-CLUSTER AMPLITUDES

The factorization of amplitudes of multiple excitations from those of elementary (single and double) excitations is only reliable when excit ation energies are nearly additive. Such deviations from additivity ma y be important, especially in solid-state physics problems. Instead of introducing many-body operators in the coupled-cluster expansion, one may take into account the effect of these deviations from the additiv ity of excitation energies by proper energy-dependent corrections to t he usual product of amplitudes. Their efficiency is illustrated on sev eral model problems where the traditional approach gives poor results. (C) 1995 John Wiley & Sons, Inc.