Molecular dynamics simulation study of the properties of doubly occupied N-2 clathrate hydrates

We have performed molecular dynamics calculations to obtain a number of pro perties of a doubly occupied structure II N-2 clathrate hydrate, in particu lar to study its behavior under higher pressures. The calculated neutron di ffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (i n either single or double occupancy) on the calculated pattern is demonstra ted and discussed. The calculated Raman spectra show that the average vibra tional frequency of the N-2 molecules decreases in going from the singly oc cupied small cages to the doubly occupied large cages and then to the singl y occupied large cages, respectively. The frequency distributions are expla ined in terms of radial distribution functions. When applying large pressur es at low temperatures, a clathrate-amorphous transition occurs for a parti ally doubly occupied clathrate. The transition occurs at about the same pre ssure as for single occupations, but the densification is larger for the la tter. In both cases, the transition is reversible. (C) 2001 American Instit ute of Physics.