Physica A 257 (1998) 526–529 Statistical thermodynamics of cluster phase transitions A. Strachan ∗ , C.O. Dorso Departamento de F sica, FCEyN, Universidad de Buenos Aires, Pabell on 1, Ciudad Universitaria, Nu˜ nez 1428, Buenos Aires, Argentina Abstract We study phase transitions of two-dimensional Lennard-Jones drops. A wide excitation energy range is studied, which encompasses the solid-like to liquid-like phase transition, the weakly evaporating liquid drop and multifragmentation. We are able to calculate the caloric curve for the whole energy range, it displays the standard plateau of the solid-like to liquid-like transition and new plateau which denotes the evaporation and multifragmentation phase transitions. We further calculate the maximum local Lyapunov exponent of the drops as a function of the energy. This curve shows a peak in the region of evaporating liquid. c  1998 Elsevier Science B.V. All rights reserved. PACS: 36.40.Ei; 36.40.Qv; 64.60.-i; 64.70.-p Keywords: Multifragmentation; Clusters; Phase transitions; Molecular dynamics 1. Introduction Phase transitions in small systems have attracted much attention during the last decade. Computer experiments showed that solid-like to liquid-like phase transition can take place in clusters of some tens of particles. This phenomenon is usually studied via the calculation of the caloric curve (T (E)) which, in the microcanonical ensemble, shows a region of negative slope denoting the phase transition [1]. Recently, the max- imum Lyapunov exponent was shown to display a jump at the transition [2]. Apart from this, small systems feature, at higher energies, a very interesting phenomena: the drop “explodes” and breaks into several intermediate mass fragments; this highly non-equilibrium process has been dubbed as multifragmentation [3,4]. In the present work we study a two-dimensional Lennard-Jones system in a wide energy range via molecular dynamics. We extend the use of the caloric curve and maximum Lyapunov exponent to study the evaporation process and multifragmentation. * Corresponding author. 0378-4371/98/$19.00 Copyright c  1998 Elsevier Science B.V. All rights reserved. PII: S0378-4371(98)00187-3