  This last Part IV is aimed at deriving relaxation rates (times) of an
off-center Li+ impurity. We follow Christov's reaction rate method to define
general rate equations in terms of the exact Mathieu eigenvalues, as well as of
harmonic-oscillator eigenvalues approximating for the energy spectrum near the
bottom of the reorientational wells. To calculate the rate in each particular
case, we derive configurational tunneling probabilities by either Mathieu
eigenfunctions or by harmonic oscillator eigenfunctions. The electron-transfer
probability is calculated by generalizing Landau-Zener's method. Typical
examples are considered and compared with experimental relaxation times in
KCl:Li+.
