SINGLE-REFERENCE THEORIES OF MOLECULAR EXCITED-STATES WITH SINGLE AND DOUBLE SUBSTITUTIONS

Several single-reference excited-state methods based on single and double substitutions are considered. Quadratic configuration interaction (QCISD) and coupled-cluster theory (CCSD) are obtained in a time-dependent linear response framework, together with the CISD method. The QCISD and CCSD transition energies are size consistent, and exact for two-electron systems. The relation between the QCISD and CCSD excited-state theories and ground-state gradient expressions is developed and employed. Calculations are reported for singlet and triplet excited states of some small molecules. CCSD and QCISD are qualitatively superior to CISD. Overall, CCSD exhibits noticeably better accuracy than QCISD, and the differences are sometimes much larger than for ground-state problems. A possible explanation is suggested.