ELECTRON CORRELATION IN THE SELF-TRAPPED HOLE AND EXCITON IN THE NACL CRYSTAL

An ab initio embedded molecular cluster method was used to find equilibrium configurations of the self-trapped hole (V-K center) and self-trapped exciton in the NaCl crystal. The results obtained in the Hartree-Fock approximation are compared with those using the Moller-Plesset second-order perturbation theory (MP2) for the calculation of the electron correlation correction to the total energy. The excitation energies for the V-K center and self-trapped exciton (STE) were calculated using the configuration interaction for single excitations (CIS) combined with the MP2 method. It is demonstrated that the Sigma band of the optical absorption spectrum of the V-K center consists of two bands. These correspond to the intramolecular electron transition in the Cl-2(-) molecular ion, and the ''host-to-molecule'' transition from the surrounding lattice anions. For the self-trapped exciton, it is found that the atomic structure of the ''off-center'' configuration of the ground state of the triplet STE is not strongly affected by taking account of the electron correlation. In particular, the off-center displacement of the center of mass of the hole component of the STE from the on-center configuration is less than half that corresonding to a pair of nearest F and H centers. The intramolecular distance in the hole component of the STE obtained in this calculation is much shorter than in previous calculations and close to that in the H center. Two Sigma polarized transitions with the energies of 3.7 and 4.02 eV were found for the hole component of the ''off-center'' STE. The correlated treatment, in contrast with the Hartree-Fock and one-electron approximations, predicts the existence of the local minimum on the adiabatic potential surface which corresponds to the ''on-center'' STE configuration (electron trapped by the V-K center). The calculated energy of optical absorption by the electron component of the triplet STE in this configuration is 0.6 eV and that of the hole component is close to that of the optical absorption of the V-K center.