Evolution of low-temperature Hg8 absorption spectrum with increased external pressure.

We have investigated different approaches to handle parallel-tempering Monte Carlo (PTMC) simulations in the isothermal-isobaric ensemble of molecular cluster/nanoparticle systems, for predicting structural phase diagram transitions. We have implemented various methodologies that consist in treating pressure implicitly, through its effect on the volume. Thus, the main problem in the simulations under non-zero pressure, becomes the volume definition of the finite non-periodic system, and we considered approaches based on the particles' coordinates. Various volume models, namely container-volume, particle-volume, average-volume, elipsoides-volume, and convex hull-volume were employed, and the required corrections for each of them in the Monte Carlo computations were introduced. Finally, we explored the effects of volume/pressure changes for all models on structural phase transitions of a test system, such as the small "ice-like" (H 2 O) 12 water cluster. The temperature and pressure dependence of the cluster's heat capacity, and energy-volume Pearson correlation coefficient was studied, phase diagrams were constructed, using a multiple-histogram method, and attempts were made to identify phase transitions to particular cluster structures. Our results show significant differences between the employed volume models, and we discuss all pressureinduced, such as solid-solid, solid-liquid and liquid-gas like, phase transformations in the present study.