An ab initio theoretical study of the optical absorption spectrum of Ni 2ϩ -doped MgO has been conducted by means of calculations in a MgO-embedded ͑NiO 6 ) 10Ϫ cluster. The calculations include long-and short-range embedding effects of electrostatic and quantum nature brought about by the MgO crystalline lattice, as well as electron correlation and spin-orbit effects within the ͑NiO 6 ) 10Ϫ cluster. The spin-orbit calculations have been performed using the spin-orbit-CI WB-AIMP method ͓Chem. Phys. Lett. 147, 597 ͑1988͒; J. Chem. Phys. 102, 8078 ͑1995͔͒ which has been recently proposed and is applied here for the first time to the field of impurities in crystals. The WB-AIMP method is extended in order to handle correlation effects which, being necessary to produce accurate energy differences between spin-free states, are not needed for the proper calculation of spin-orbit couplings. The extension of the WB-AIMP method, which is also aimed at keeping the size of the spin-orbit-CI within reasonable limits, is based on the use of spin-free-state shifting operators. It is shown that the unreasonable spin-orbit splittings obtained for MgO:Ni 2ϩ in spin-orbit-CI calculations correlating only 8 electrons become correct when the proposed extension is applied, so that the same CI space is used but energy corrections due to correlating up to 26 electrons are included. The results of the ligand field spectrum of MgO:Ni 2ϩ show good overall agreement with the experimental measurements and a reassignment of the observed E g (b 3 T 1g ) excited state is proposed and discussed.

In this paper we present nonrelativistic and relativistic core ab initio model potentials ͑AIMPs͒ and valence basis sets for La and the third-series transition metal elements. The relativistic AIMPs are derived from atomic Cowan-Griffin calculations; they are made of a spin-free part and a one-electron spin-orbit operator according to Wood and Boring. The core potentials correspond to the 62-electron core ͓Cd,4f ͔. The valence basis sets are optimized and spin-orbit corrected. We present monitoring spin-free calculations on the atoms, singly ionized ions and monohydrides of the ten elements, which show a good performance overall. A spin-free-state-shifted spin-orbit-configuration interaction calculation on Pt, which uses empirical spin-free data and which is expected to be essentially free from spin-free deficiencies, points out that the quality of the spin-orbit operators is very good.

This work presents a systematic test of the performance of a spin-orbit operator founded upon the Wood-Boring-based ab initio model potential method ͓J. Chem. Phys. 102, 8078 ͑1995͔͒. Assuming a separability of the problem into a spin-free correlation treatment and a spin-orbit calculation part, this aim can be reached. We shall show in this publication both the separability and the high level of quality of the spin-orbit operator applying our method to the even spectrum of Ir ϩ . We shall treat the spin-orbit part by means of the above mentioned spin-orbit operator and cope with the spin-free correlation problem through introducing a spin-free-state-shifting operator, shifting the spin-free energies to empirical values obtained from experiment. The quality of the spin-orbit operator is very high, actually better than estimated in previous calculations which were contaminated by an insufficient treatment of correlation. The procedure established is most efficient: Spin-free-stateshifted spin-orbit CI calculations employing a space of the significant reference configurations plus single excitations lead to very reliable spin-orbit splittings provided that the spin-free states are calculated at a high level of quality.

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