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Issue 45, 2014
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Nucleation free-energy barriers with Hybrid Monte-Carlo/Umbrella Sampling

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The aim of this work is to evaluate nucleation free-energy barriers using molecular dynamics (MD). More specifically, we use a combination of Hybrid Monte Carlo (HMC) and an Umbrella Sampling scheme, and compute the crystallisation barrier of NaCl from its melt. Firstly the convergence and performance of HMC for different time-steps and the number of MD steps within a HMC cycle are assessed. The calculated potential energies and densities converge regardless of the chosen time-step. However the acceptance ratio of the Metropolis step within the HMC scheme strongly depends on the time-step and affects the performance. It is shown that the acceptance ratio is close to 100% for time-steps of the order of those commonly used in molecular dynamics runs. We then explore the results obtained with a “non-Metropolised” version of HMC where the MD trajectories are always accepted (omitting the Metropolis criteria) and conclude that they are satisfactory for time-steps below 5 fs. Next, HMC is combined with Umbrella Sampling (HMC/US) to compute the nucleation free-energy for both the standard and the “non-Metropolised” HMC (using a small time-step) and in both cases find excellent agreement with the reported values. To conclude, we explore approximations to the HMC/US technique implementing HMC with isothermal–isobaric MD trajectories. The computed nucleation free-energy curve is coincident, within the statistical error, with previous calculations.

Graphical abstract: Nucleation free-energy barriers with Hybrid Monte-Carlo/Umbrella Sampling

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Publication details

The article was received on 26 Jun 2014, accepted on 19 Sep 2014 and first published on 17 Oct 2014

Article type: Paper
DOI: 10.1039/C4CP02817A
Citation: Phys. Chem. Chem. Phys., 2014,16, 24913-24919
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    Nucleation free-energy barriers with Hybrid Monte-Carlo/Umbrella Sampling

    M. A. Gonzalez, E. Sanz, C. McBride, J. L. F. Abascal, C. Vega and C. Valeriani, Phys. Chem. Chem. Phys., 2014, 16, 24913
    DOI: 10.1039/C4CP02817A

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