AN AB-INITIO MOLECULAR-ORBITAL APPROACH TO ELECTRONIC-STRUCTURES OF THE F-CENTER IN NACL - EFFECTS OF BASIS-SET AND CLUSTER-SIZE

NaCl clusters with a Cl- ion vacancy, Na-6(6+), Na.6Cl126-, Na14Cl122+, and Na14Cl184-, are calculated by ab initio molecular orbital calculations to investigate electronic states of F centers in alkali halides. The model clusters are embedded in a point charge field. Description of ''1s'' ground and ''2s'' and ''2p'' excited states of an electron trapped by the vacancy is discussed concerning to basis set and cluster size. The 1s and 2p wave functions of the F-center electron are efficiently described by one s Gaussian-type function placed on the vacancy center and one p Gaussian-type functions placed on the nearest neighbor Na ions when Na 3s is of double zeta quality. The 1s-2p excitation energy is in good agreement with the experimental data. As regards the electron binding energies and the orbital size the 1s wave function almost converges in the cluster Na6Cl126- but the 2p wave function does not converge even in the largest cluster Na14Cl184-.