An analysis and implementation of a general coupled cluster approach to excitation energies with application to the B-2 molecule

A general scheme is presented for the calculation of excitation energies us ing the standard coupled cluster hierarchy and a simple implementation is d escribed for the higher standard models. An error analysis is performed to find to what order excitation energies in different coupled cluster models are correct. The analysis includes both the standard coupled cluster hierar chy as well as the approximate models and considers excitations to states t hat are dominated by one, two, and three electron replacements compared to the reference state. Calculations are presented up to the quadruple excitat ion level for the open shell B-2 molecule using an excited closed shell sta te as reference state to emphasize the usefulness of the order analysis. Th e coupled cluster excitation energies are compared to full configuration in teraction results. (C) 2001 American Institute of Physics.