CALCULATING EXCITATION-ENERGIES WITH THE HELP OF CUMULANTS

Recently, it has been shown that the ground-state energy Of a quantum many-body system can be written in terms of cumulants. In this article, we show that the energies of excited states can be expressed similarly. These representations are suitable for various approximation schemes, e.g., projection techniques. The explicit use of cumulants ensures size consistency in all approximations. The theory is then applied to the computation of the energy bands of a semiconductor with a diamond structure. As we focus especially on the effects of electron correlations, we consider a simplified model and rederive results obtained previously by the variational local ansatz method. (C) 1994 John Wiley & Sons, Inc.