SAT-TMMC: Liquid-vapor coexistence properties - Linear-Force Shifted Potential at 4.0σ

Liquid-vapor coexistence properties obtained by grand-canonical transition-matrix Monte Carlo and histogram re-weighting over the reduced temperature range 0.60 to 1.20 at increments of 0.05. Mean values of the saturation pressure, density, potential energy per molecule, and activity (chemical potential- see below) for each phase are reported.

Remarks:

Uncertainties were obtained from five independent simulations and represent 95% confidence limits based on a standard t statistic. Liquid-vapor coexistence was determined by adjusting the activity such that the pressures of the liquid and vapor phases were equal. Here, the pressure is not the conventional virial pressure [2,3] but is the actual thermodynamic pressure, based on the fact that the absolute free energies can be obtained from the distributions determined from simulation [4]. Alternative methods, for example Gibbs-ensemble Monte Carlo and combination grand-canonical Monte Carlo and histogram re-weighting, can be used to determine liquid-vapor coexistence. A review of standard methods of phase equilibria simulations can be found in Ref. 5.

As introduced in Refs. 2 and 3, the activity, z, is defined as

$$ z = \dfrac{ \exp\left( \beta \mu \right)}{\lambda^3}$$

where Λ is the de Broglie wavelength, β = 1/(k_BT) (where k_B is Boltzmann's constant), and μ is the chemical potential. It is sometimes more convenient to work with ln z in the simulations and in post-processing. (NOTE: The reported activity is dimensionless, having been scaled by the LJ length cubed.)

Phase-coexistence energies were obtained by determining the mean potential energy at a given value of N for an additional 40 billion MC trials. Combining this information with the particle number probability distribution, the mean potential energy of the coexisting phases can be calculated [6].

For the Lennard-Jones fluid, linear force shifted at 4.0σ, the critical properties were estimated to be T_c*=1.207, ρ_c*=0.319, and p_c*=0.112. Estimates were found via rectilinear diameter analysis of TMMC data computed with V*=729 close to the critical point [7]. (Finite-size scaling analysis has not been completed, so these critical properties should be taken simply as estimates.)

References

1. J. R. Errington, J. Chem. Phys. 118, 9915 (2003).
2. M. P. Allen and D. J. Tildesley, Computer Simulation of Liquids (Oxford University Press, New York, 1989).
3. D. Frenkel and B. Smit, Understanding Molecular Simulation, 2nd ed. (Academic, San Diego, 2002)., pp.37-38.
4. J. R. Errington and A. Z. Panagiotopoulos, J. Chem. Phys., 109, 1093 (1998).
5. A. Z. Panagiotopoulos, J. Phys.: Condens. Matter, 12, R25-R52, (2000).
6. J. R. Errington and V. K. Shen, J. Chem. Phys., 123, 164103 (2005).
7. B. Smit and C. P. Williams, J. Phys.: Condens. Matter, 2, 4281-4288 (1990).
8. V. K. Shen and D. W. Siderius, J. Chem. Phys., 140, 244106, 2014.
9. V. K. Shen and J. R. Errington, JPC B 108, 19595, 2004.
10. V. K. Shen and J. R. Errington, JCP 122, 064508, 2005.
11. V. K. Shen, R. D. Mountain, and J. R. Errington, JPC B 111, 6198, 2007.
12. D. W. Siderius and V. K. Shen, JPC C 117, 5681, 2013.