Abstract
The coupled-cluster method of electronic-structure calculation is briefly introduced and examined as to its dependence upon the choice of reference state. It is found that the method depends relatively weakly on the reference state if single-particle “clusters” are included in the calculations. This fact makes it reasonable to combine coupled-cluster calculations of ground and excited states, based on the same reference wave function, to obtain an equation for the excitation energy. This excitation-energy equation is of nearly the same form as that obtained by the “equations of motion” approach, but contains additional terms which should improve the description of orbital-relaxation and state-dependent correlation effects.
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