Multireference and relativistic effects in NiH

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Abstract
Large multireference CI calculations have been performed for the ground state of NiH. The effects of relativity were investigated using both a variational (no pair) theory and perturbation theory. The largest CI calculations included up to 29 reference states and were performed at the contracted CI level. The calculated and experimental results (within parentheses) are for Re 2.76 a0 (2.76 a0), for ωe 1997 cm−1 (2003 cm−1) and for the dipole moment μ 2.32 D (2.4±0.1 D). The effects of relativity are −0.03 a0, +60 cm−1 and −0.3 D, respectively. The effects of going from a reference selection threshold of 0.05 to a selection threshold of 0.02 and a different set of CASSCF orbitals was before applying Davidson’s correction +0.07 a0 for Re and about +30 cm−1 for ωe, and after applying this correction +0.06 a0 and −80 cm−1, respectively. The most accurate results for the dipole moment was obtained using the multireference ACPF method.

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