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Classical and quantum calculations of the temperature dependence of the free energy of argon

Xu, W; Horsfield, AP; Wearing, D; Lee, PD; (2017) Classical and quantum calculations of the temperature dependence of the free energy of argon. Computational Materials Science , 144 pp. 36-41. 10.1016/j.commatsci.2017.12.001. Green open access

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Abstract

The free energy is central to statistical mechanics and thermodynamics, and its accurate calculation via. computational modelling is important for a large number of applications, especially when its experimental value is hard to obtain. Several established and general methods for calculating the Helmholtz free energy across different length scales, including continuum, atomistic and quantum mechanical, are compared and analyzed. A computational approach is then proposed to calculate the temperature dependences of internal energy and absolute Helmholtz free energy for solid and liquid phases with the coupling of thermodynamic integration (TI) and harmonic approximation calculations from both classical molecular dynamics (MD) and density functional theory (DFT). We use the Lennard-Jones system as an example (i.e. argon) for the demonstration of the approach. It is observed that the free energy transits smoothly from being describable by the harmonic approximation to including anharmonic effects at a transition temperature around 0.56 T m ; below this temperature, the quantum behavior of atoms is important. At higher temperatures (T > 0.56 T m ), the TI and harmonic approximation results for the Helmholtz free energy functions become increasingly divergent with the increase of temperature. This work demonstrates that a multiscale approach employing TI, MD, and DFT can provide accurate calculations of the temperature dependence of absolute Helmholtz free energy for both solid and liquid phases.

Type: Article
Title: Classical and quantum calculations of the temperature dependence of the free energy of argon
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.commatsci.2017.12.001
Publisher version: https://doi.org/10.1016/j.commatsci.2017.12.001
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Thermodynamic integration, Harmonic approximation, Molecular dynamics, Quantum mechanics, Helmholtz free energy, Argon
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10049189
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