Effects of isotope disorder on energies and lifetimes of phonons in germanium

based on the coherent potential approximation and on large supercells, respectively. For the mass variance of our sample, the two methods lead to rather similar results that agree with the experimental data. This agreement is marginally better for the supercell calculations. Calculations performed for larger mass variances g revealed, however, that the two approaches yield significantly different results when g is larger than 3 310 23 . @S0163-1829~98!02938-5#