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

Abstract

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 E rel. The electronic ground state of NiCN is 2Δ i and the equilibrium geometry is linear with bond lengths of r e(Ni–C) = 1.8141 Å and r e(C–N) = 1.1665 Å. According to multi-reference averaged quadratic coupled-cluster theory (MR-AQCC) with E rel, the first excited state is 2Σ +, separated from the ground state by 446 cm −1, the second and third excited states are 2Π i (at 1594 cm −1) and 4Δ i (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 ∼ 2 Δ 5 / 2 ) , 793 ( 2 Σ 1 / 2 + ) , 891 ( 2Δ 3/2), 2321 ( 2Π 3/2), and 2773 ( 2Π 1/2). 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 2 Σ 1 / 2 + state.