PATH-INTEGRAL SIMULATIONS OF MIXED PARA-D-2 AND ORTHO-D-2 CLUSTERS - THE ORIENTATIONAL EFFECTS

A recently suggested computational scheme [Buch and Devlin, J. Chem. Phys. 98, 4195 (1993)] is extended to describe orientational phenomena in systems containing assemblies of odd-J and even-J hydrogen and deuterium molecules. Nuclear symmetry and rotational effects are incorporated in the path integral Monte Carlo algorithm. The scheme is employed in a study of the (para-D-2)(3)(ortho-D-2)(10) and (para-D-2)(13) clusters in the 1-3 K temperature range. In the (para-D-2)(3)(ortho-D-2)(10) cluster at 1 K, the three J=1 molecules form a ''cluster within cluster,'' held. together by the anisotropic quadrupole-quadrupole interactions. At 3 K the three molecule J=1 cluster is dissolved within the (D-2)(13) cluster. The orientational structure of the (para-D-2)(13) cluster at 1 K is composed of two distinct distorted-T pair configurations of neighboring J=1 molecules. Upon heating to 3 K the orientational structure changes, and a clear separation between the two pair geometries is no longer obtained. Moreover at 3 K thermal distortion of the cluster structure from icosahedral relaxes the extent of orientational frustration of the system, and thus contributes to the lowering of the rotational energy.