“…Empirical 6-exp atom-atom potential-energy models proved to be very reliable (Pertsin & Kitaigorodsky, 1987). The method has recently been applied to silicates (Pilati, Bianchi & Gramaccioli, 1990), and the effects of molecular non-rigidity have also been considered (Gramaccioli, Filippini & Simonetta, 1982;Gramaccioli & Filippini, 1983;Filippini, 1990).…”