A Comparison of Density Functional and Coupled-Cluster Theories for the Equilibrium Properties of Valence Excited Electronic States: Spectroscopic Constants of Diatomics

The reliability of density functional theory and other electronic structure methods is examined for anharmonicities and spectroscopic constants of low-lying valence electronic excited states. The equilibrium bond length re, harmonic vibrational frequency ωe, rotational constant Be, centrifugal distortion constant D(bar)e , and vibration-rotation interaction constant αe, have been determined for low-lying singlet excited states of BF, CO, N2, CH+, and NO+ . Predictions using configuration interaction singles, equation-of-motion coupled-cluster singles and doubles, and various time-dependent density functional methods have been made using the 6-31G*, aug-cc-pVDZ, and aug-cc-pVTZ basis sets and compared to experimental values. Time- dependent density functional theory compares favorably to equation-of-motion coupled-cluster theory for the properties considered, but errors are substantially larger for excited states than for ground states.