Semiempirical Molecular-Orbital Treatment of theUBand in LiF:H−

Calculations using the semiempirical self-consistent-field procedure known as complete neglect of differential overlap (CNDO) have been made on a collection of lithium and fluorine atoms in an attempt to generate a model of the $U$ center. The system consists of 27 atoms arranged in a fcc lattice. The central fluorine atom is replaced by a hydrogen atom, simulating the $U$ center. Calculation of the CNDO eigenvalue spectrum for this "molecular" model shows that the essential features of this defect are represented very well and that the value obtained for the $U$-band excitation energy compares well with empirical and other theoretical data.