  Extending earlier work on a vibronic theory of the FA centers in alkali
halides, the reorientation is now considered of an off-center Li+ impurity,
either isolated or near an F center. We derive analytically periodic potential
energy barriers between metastable reorientational off-center sites, the
barriers hindering the impurity rotation around the normal lattice site.
Applying to a specific model, electron-vibrational mode coupling constants are
calculated up to 3rd order of the expansion of the coupling energy in the T1u
mode coordinates. The 3rd order coupling brings about additional
renormalization of the reorientation controlling vibrational frequency.
