ENERGIES AND STRUCTURES OF ROTATING ARGON CLUSTERS - ANALYTIC DESCRIPTIONS AND NUMERICAL SIMULATIONS

摘要

The dependence of the rotational energy of small argon clusters on the magnitude and direction of their rotational angular momenta is obtained by two different methods, namely, by analytic descriptions parametric in structural variables (centrifugal displacements) and by classical simulations carried out in rotating frames so that rotational angular momenta are conserved. Potential energies are taken as additive Ar, pair potentials [R. A. Aziz, J. Chem. Phys. 99, 4518 (1993)], augmented in some cases by three-body Axilrod-Teller interactions, thus complementing our earlier studies of rare-gas clusters modeled by additive Lennard-Jones oscillator (LJO) pair potentials [L. L. Lohr and C. H. Huben, J. Chem. Phys. 99, 6369 (1993)]. Quartic and sextic spectroscopic constants are found to be approximately 10% smaller when the Aziz pair potential is used, reflecting its greater stiffness as compared to the LJO potential. The sign of the sextic tensor coefficient for both tetrahedral Ar, and octahedral Ar-6 is such that for sufficiently high J the C-2v (or D-2h) structures with J parallel to a pseudo-C-2 (or true C-2) axis (saddle points on the rotational energy surface at low J) become local energy maxima, the D-2d (or D-4h) structures with J parallel to an S-4 (or C-4) axis representing the energy minima. The trigonal bipyramidal cluster Ar-4 resembles both Ar-3 and Ar-4 in its rotational characteristics but with reduced manifestations of nonrigidity. As found with an LJO pair potential [D. H. Li and J. Jellinek, Z. Phys. D 12, 177 (1989)], the icosahedral Ar-13 cluster displays a very slight preference for D-3d structures with J parallel to a C-3 axis, while the D-5d structures with J parallel to a C-5 axis are energy maxima and the D-2h structures with J parallel to a C-2 axis are saddle points on the rotational energy surface. The scalar quartic spectroscopic coefficient for Ar-13 is found to be 2.15 x 10(-4) times that for the reference diatomic Ar-2. A variety of structural instabilities are described for Ar-13 clusters with very high rotational energies. (C) 1996 John Wiley & Sons, Inc. [References: 30]