The atomic correlation terms necessary to lead to an accurate 4s 23d s-4s 13d 9 separation for the Ni atom have been incorporated into all-electron MC SCF/CI calculations for the X 2A state of NiH. The calculated potential curve properties are significantly improved compared to calculations which dissociate to Hartree-Fock atoms.
Bagus and Bjorkman [1 ] (BB)recently reported on calculations for NiH. They used a large STO basis and performed HF, limited MC SCF and CI restricted to the four o valence electrons. They observed that the HF bond length was too long by ~-~43.15 bohr and there was very little binding energy. Their limited MC SCF calculation improved the De, but made the bond length even longer, while their simple 4el CI shortened the bond length into good agreement with experiment. They were also able to predict that only one of the two states at -~15000 cm -1 was a 2A state. More recently Siegbahn and Blomberg [2] (SB), using a smaller GTO basis set have also investigated NiH. They performed extensive MC SCF and large-scale CI. Their observations and conclusions are in agreement with the smaller calculations of BB.
We have also performed large MC SCF and extensive CI calculations on NiH (using a GTO basis set), but unlike BB or SB, we have restricted ourselves to the ground (2A) state. We have studied the effect on the molecular properties of NiH of improving the description of the low-lying atomic states of the Ni atom. This leads to an understanding of the bond shortening at the CI level.
As noted by BB, the bonding in NiH primarily arises from 3d 94s 1. The experimental separation of the * The authors were supported under NASA Contracts NAS 2-10687 and NAS 2-10763.