“…In fact, Raman and infrared active frequencies, isotropic and anisotropic temperature factors, and thermodynamic functions have been derived in this way in good agreement with the experimental values, starting from Lennard-Jones or 6-exp functions like Williams IVa (Williams, 1967), or some sets proposed by Kitaigorodskii (1966) or others (Pawley, 1967;Taddei, Bonadeo, Marzocchi & Califano, 1973;Gramaccioli, Simonetta & Suffritti, 1973;Filippini, Gramaccioli, Simonetta & Suffritti, 1973, 1974a,b, 1975a. Even phonon dispersion curves calculated in this way are remarkably close to the experimental results, at least in the few cases which have been reported so far (Lutz & H~ilg, 1970;Pawley et al, 1980;Natkaniec et al, 1980). The agreement is indeed surprising, because most of these potentials have not been derived from fitting experimental frequencies.…”