A comparison of single reference methods for characterizing stationary points of excited state potential energy surfaces

The accuracy of geometries, vibrational frequencies and dipole moments of stationary points on excited state potential energy surfaces is assessed for three single reference excited state theories—configuration interaction (CIS), a perturbative doubles correlation correction to CIS, termed CIS(D), and equation-of-motion coupled cluster theory with single and double substitutions (EOM-CCSD). Two groups of systems are studied: the diatomic molecules H2, BH, BF, C2, CO, and N2; and the lowest singlet excited states of ammonia, formaldehyde and acetylene. The calculations demonstrate that CIS systematically underestimates bond lengths and overestimates frequencies and dipole moments, a pattern often associated with the Hartree–Fock method for ground states. CIS(D) fails to provide a systematic improvement to CIS for all geometries and frequencies, often overestimating correlation corrections. EOM-CCSD, by contrast, performs significantly better than CIS for all properties considered.