Assessment of isobaric-isothermal (NPT) simulations for finite systems

A compilation and an implementation of different methodologies to simulate NPT ensembles on finite systems is presented. In general, the methods discussed can be classified in two different groups depending on how the external pressure is applied to the system. The first approach is based on including the pressure with its conjugate thermodynamical variable, the volume, in the Lagrangian of the system. For this group four different volume definitions were considered and we assess their validity by studying the structural properties of small systems as function of pressure. In particular, we focus on the stability of the C60 molecule as well as the amorphization process of a diamond-like cluster under pressure. In the second group, the finite system (C60) is embedded in a classical fluid which serves as a pressure reservoir. We take the latter method as reference because it is closest to the experimental situation. The difficulties and the regimes where these methods can be used are also discussed.