Temperature-quench Molecular Dynamics Simulations for Fluid Phase Equilibria

Abstract

A method for locating fluid phase equilibria by means of a single canonical molecular dynamics simulation is evaluated. The temperature-quench molecular dynamics (TQMD) method consists of quenching an initially homogeneous one-phase fluid system to a lower temperature where it is mechanically and thermodynamically unstable. After a short transient, domains of coexisting phases form, which quickly acquire equilibrium-like properties. A suitable analysis of the coexisting domains in terms of local densities, compositions, or some other order parameter gives the phase equilibrium properties. We show how, contrary to expectations, one need not wait until a full global equilibration (planar interface) is resolved to obtain the correct results. As examples, the phase diagram of a cut and shift (5σ) Lennard–Jones fluid, the pressure-composition diagram of a Lennard–Jones mixture presenting three-phase vapor–liquid–liquid equilibria and the saturated liquid densities of eicosane using a united atom representation are determined. Comments and comparisons are made with the most commonly used methods such as Gibbs Ensemble Monte Carlo and volume expansion molecular dynamics (VEMD), finding TQMD to be a suitable alternative, especially for complex molecules and/or high densities.

Keywords:

• Fluid phase equilibria
• Vapor–liquid equilibrium
• Vapor–liquid–liquid equilibria
• Lennard–Jones
• Eicosane

Acknowledgements

We thank Prof Lev D. Gelb for lengthy discussions and helpful comments.

Notes

Tel.: [phone]. Fax: [phone]. E-mail: [email protected]

http://www.chemistry.wustl.edu/∼ gelb/lj2/

http://www.cse.clrc.ac.uk/msi/software/DL_POLY/

http://www.car.labf.usb.ve/