The low-lying electronic states of the NiH2 molecule were investigated by using the MCQDPT2 method. In order to accurately describe the strong correlation derived from the nickel 3d9 super-configuration, a set of diffuse secondary 3d' orbitals were included in the active space, yielding a large active space of 12 electrons in 13 orbitals. It is shown that the absolute minimum energy configuration of NiH2 is bent, in agreement with the experimental observation. The global ground state is 1A1 (or A1 in the spin-orbit coupling case), whereas the lowest linear state is 3Deltag (or 3g). Some other cheaper single-configurational and multi-configurational methods were also used to study both states, and their shortcomings are discussed. Our theoretical results suggest that the arrangement of the experimental frequencies of NiH2 and NiD2 may be incorrect.
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