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 calcul ations 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 cluste r size. The 1s and 2p wave functions of the F-center electron are effi ciently described by one s Gaussian-type function placed on the vacanc y center and one p Gaussian-type functions placed on the nearest neigh bor 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 th e electron binding energies and the orbital size the 1s wave function almost converges in the cluster Na6Cl126- but the 2p wave function doe s not converge even in the largest cluster Na14Cl184-.