Equation-of-motion coupled-cluster calculations of excitation energies. The challenge of ozone

In this paper, we report the calculation of singlet and triplet excitation energies of the ozone molecule using equation-of-motion coupled-cluster methods (EOM-CC). This molecule presents a formidable challenge to these methods, owing to significant multi-reference character in its ground state and the presence of several low-energy doubly excited states. We have used the singles-and-doubles approximation (EOM-CCSD), non-iterative inclusion of triple excitations, and the iterative inclusion of triple excitations by EOM-CCSDT-1c and EOM-CCSDT-3. Some single-reference CCSDT-3 calculations were also performed on some of the triplet states. With the EOM-CCSDT-1c and EOM-CCSDT-3 methods (to some extent the EOM-CCSD method),the results for singly excited states are quite satisfactory and competitive with prior work. The iterative triples methods do not perform as well for the doubly excited states. However, they substantially reduce the errors for these states given by EOM-CCSD. (C) 1999 Elsevier Science B.V. All rights reserved.