The lowest singlet and triplet electronic states of NiC revised

A multireference configuration interaction (MRCI) approach, based on complete-active-space self-consistent-field (CASSCF) wave functions, with extended basis sets such as Watchers’ (14s 11p 6d 3f)/[8s 6p 4d 1f] for nickel and correlation-consistent polarized-valence triple-zeta (cc-pVTZ) for carbon, were employed to revise the lowest-lying singlet and triplet electronic states of NiC. The energetic ordering (Te values in parenthesis) of the electronic states, X1Σ+, a3Π (7882 cm−1), A1Π (9427 cm−1), B1Φ (10 156 cm−1), b3Φ (10 689 cm−1), C3Σ+ (10 835 cm−1), C1Δ (14 352 cm−1), and d3Δ (14 363 cm−1), was found to be in variance with that obtained previously at the CASSCF level, re-emphasizing the need of including nondynamical and dynamic correlations effects into the wave function of systems containing transition metals.