Statistical mechanics and morphology of very small atomic clusters

Abstract

Mechanical and thermodynamic systems of a very few atoms (N≤ 100) interacting through central-forces are considered in the light of their role in an ideal model for “precipitation”. We discuss the stability of such systems at low energies and present detailed numerical investigations of the potential-energy surfaces of “clusters” of up to thirteen atoms interacting through Lennard-Jones and Morse potentials. Our main results comprise what we believe to be an almost exhaustive survey of the distinct minima available to systems of this size for the potentials used. Each such stable structure obtained is vibrationally and rotationally analysed and its symmetry is examined. The most striking feature of these results is the extreme sensitivity of the number of possible stable configurations to the range and softness of the pair potential. Thus, of no fewer than 988 minima for 13 Lennard-Jones atoms, only some 36 are supported by the (α= 3) Morse potential. The minima available are also classified geometrically and it is shown that non-crystallographic configurations predominate in structures of both greatest and least binding energy. A preliminary account of the statistical mechanics of cluster systems based on the rigid-rotor/harmonic oscillator approximation is given.