MICROCANONICAL TEMPERATURE AND HEAT-CAPACITY COMPUTATION OF LENNARD-JONES CLUSTERS UNDER ISOERGIC MOLECULAR-DYNAMICS SIMULATION

The cumulated distribution of velocity components of atoms of a cluste r in a constant energy classical molecular dynamics simulation is foun d to fit very well with Maxwellian distribution. This enables, with ca refully prepared initial configurations, a cluster as small as contain ing just four atoms to be viewed to constitute a canonical ensemble in itself. In addition, the statistical distribution of velocity compone nts provides an unambiguous, independent and robust method of obtainin g the thermodynamic temperature of an isolated system. The temperature thus obtained differs from the conventionally used kinetic temperatur e by a fixed factor of (3N-6)/(3N), implications of which are discusse d. A new procedure of sampling the configurational energy landscape is introduced. and the ''heat capacity'' curve computed using this metho d--which is actually the second moment of the potential energy fluctua tions as a function of cluster kinetic energy--exhibits its usefulness by clearly demonstrating the two stage melting of Ar-55. (C) 1998 Ame rican Institute of Physics. [S0021-9606(98)50815-X].