Ab initio molecular orbital study of ground and low-lying electronic states of NiCN

The electronic ground state and some low-lying excited states of NiCN have been studied by ab initio multi-reference single and double excitation configuration interaction calculations (MR-SDCI) with Davidson's correction Q and Cowan-Griffin's relativistic corrections Erel. The electronic ground state of NiCN is 2Δi and the equilibrium geometry is linear with bond lengths of re(Ni-C) = 1.8141 A and re(C-N) = 1.1665 A. According to multi-reference averaged quadratic coupled-cluster theory (MR-AQCC) with Erel, the first excited state is 2Σ+, separated from the ground state by 446 cm-1, the second and third excited states are 2Πi (at 1594 cm-1) and 4Δi (at 8930 cm-1), respectively, and other quartet states follow. The relative stabilities and the molecular properties of the low- and high-spin states having the same electronic total angular momentum are shown to be determined by dynamical electron correlation effects. A spin-orbit interaction scheme for the lowest doublet manifold is proposed. The predicted term values (in cm-1) are 0 (X∼2Δ5/2), 793 (2Σ1/2+), 891 (2Δ3/2), 2321 (2Π3/2), and 2773 (2Π1/2). Thus, an unassigned state reported by Kingston et al. [C.T. Kingston, A.J. Merer, T.D. Varberg, J. Mol. Spectrosc. 215 (2002) 106] at 755 cm-1 could be the 1 2Σ1/2+ state