AB-INITIO MODEL POTENTIAL CALCULATIONS ON THE ELECTRONIC-SPECTRUM OF NI2-DOPED MGO INCLUDING CORRELATION, SPIN-ORBIT AND EMBEDDING EFFECTS()

An ab initio theoretical study of the optical absorption spectrum of N i2+-doped MgO has been conducted by means of calculations in a MgO-emb edded (NiO6)(10-) cluster. The calculations include long- and short-ra nge embedding effects of electrostatic and quantum nature brought abou t by the MgO crystalline lattice, as well as electron correlation and spin-orbit effects within the (NiO6)(10-) cluster. The spin-orbit calc ulations have been performed using the spin-orbit-CI WB-AIMP method [C hem. Phys. Lett. 147, 597 (1988); J. Chem. Phys. 102, 8078 (1995)] whi ch has been recently proposed and is applied here for the first time t o the field of impurities in crystals. The WB-AIMP method is extended in order to handle correlation effects which, being necessary to produ ce accurate energy differences between spin-free states, are not neede d 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 sp in-orbit-CI within reasonable limits, is based on the use of spin-free -state shifting operators. It is shown that the unreasonable spin-orbi t splittings obtained for MgO:Ni2+ in spin-orbit-CI calculations corre lating 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 Lig and field spectrum of MgO:Ni2+ show good overall agreement with the ex perimental measurements and a reassignment of the observed E(g)(b(3)T( 1g)) excited state is proposed and discussed. (C) 1996 American Instit ute of Physics.