Finite Systems under Pressure: Assessing Volume Definition Models from Parallel-Tempering Monte Carlo Simulations

Abstract

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.

Computational investigations of stable multiple-cage-occupancy He clathrate-like hydrostructures

“…On the one hand, such findings are in line with available experimental and theoretical predictions, indicating that the analysis of the fundamental entities (such as the building block clathrate-like cages) may be representative enough to get a first idea of the stability of such inclusion compounds under specific thermodynamic conditions, although several methodological limitations, such as the pressure control on finite size systems, [104][105][106] should be further examined. On the other hand, current investigations involve exploration of the multiple cage occupancy effects by means of DFT-D computations in periodic crystals, in order to check if the predictions observed here on stability and He clustering inside the clathrate-like cages are also fulfilled in He@sII crystal structures.…”

Delving into guest-free and He-filled sI and sII clathrate hydrates: a first-principles computational study

“…26,27 The incorporation of enthalpy and entropy effects could determine whether the calculated stabilization energies are related to observed phase diagram transformations. 13,33,67,70,106,107 Understanding the underlying factors involved in the stability of such hydrates allows the extension of this work in this direction. Thus the next step consists to consider multiple cage occupancy similar to those established by diffraction experiments 23 with up to 4 He atoms in the large sII cages and one guest in the small ones, for a more direct comparison with measurements on structural properties.…”

Thermodynamics of small mercury clusters and the role of electronically excited states: a case study on Hg13

“…The cluster volume, , has been calculated via a quick-hull algorithm 24 within which the space domain occupied by the cluster has been approximated by the smallest convex envelope of the cluster represented as a set of spheres centered at atomic positions 25 and with radii equal to the van der Waals radius of the Hg atom (1.55 Å 26 ).…”