Molecular-dynamics simulations of binary structure II hydrogen and tetrahydrofurane clathrates

The binary structure II hydrogen and tetrahydrofurane (THF) clathrates are studied with molecular-dynamics simulations. Simulations are done at pressures of 120 and 1.013 bars for temperatures ranging from 100 to 273 K. For the small cages of the structure II unit cell, H-2 guest molecule occupancies of 0, 16 (single occupancy), and 32 (double occupancy) are considered. THF occupancies of 0-8 in the large cages are studied. For cases in which THF does not occupy all large cages in a unit cell, the remaining large cages can be occupied with sets of four H-2 guest molecules. The unit-cell volumes and configurational energies are compared in the different occupancy cases. Increasing the small cage occupancy leads to an increase in the unit-cell volume and thermal-expansion coefficient. Among simulations with the same small cage occupancy, those with the large cages containing 4H(2) guests have the largest volumes. The THF guest molecules have a stabilizing effect on the clathrate and the configurational energy of the unit cell decreases linearly as the THF content increases. For binary THF+H-2 clathrates, the substitution of the THF molecules in the large cages with sets of 4H(2) molecules increases the configurational energy. For the binary clathrates, various combinations of THF and H-2 occupancies have similar configurational energies. (c) 2006 American Institute of Physics.