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 H-2, BH, BE C-2, CO, and N-2; 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. (C) 1995 American Institute of Physics.