Finite temperature properties of clusters by replica exchange metadynamics: the water nonamer

TL;DR

This paper presents a novel enhanced sampling method, replica exchange metadynamics, for accurately calculating the thermal properties and free energy landscapes of nanoclusters, demonstrated on the water nonamer.

Contribution

The paper introduces replica exchange metadynamics, an improved technique that automatically includes all entropy sources in cluster free energy calculations.

Findings

The second relevant structure of water nonamer has higher configurational entropy.

Free energy crossover occurs between structures at higher temperatures.

Method accurately captures equilibrium occupancy of cluster structures.

Abstract

We introduce an approach for the accurate calculation of thermal properties of classical nanoclusters. Based on a recently developed enhanced sampling technique, replica exchange metadynamics, the method yields the true free energy of each relevant cluster structure, directly sampling its basin and measuring its occupancy in full equilibrium. All entropy sources, whether vibrational, rotational anharmonic and especially configurational -- the latter often forgotten in many cluster studies -- are automatically included. For the present demonstration we choose the water nonamer (H2O)9, an extremely simple cluster which nonetheless displays a sufficient complexity and interesting physics in its relevant structure spectrum. Within a standard TIP4P potential description of water, we find that the nonamer second relevant structure possesses a higher configurational entropy than the first, so that the two free energies surprisingly cross for increasing temperature.