Ab initio equation of state for gaseous and supercritical argon based on the virial expansion


B. Jäger1, R. Hellmann1 and E. Bich1

1Institute of Chemisty, University of Rostock, Rostock, Germany

Keywords: ab initio calculations, equation of state
property: virial coefficients
material: argon

Thermodynamic properties of industrially relevant fluids are usually predicted utilizing empirical correlations or corresponding simulation techniques. In the course of increasing computational resources it is now found practicable to compute the equations of state for gaseous and supercritical fluids with high accuracy solely from theory. For argon experimental data with very low uncertainties are available from the literature, hence, it was chosen as model substance.

New two-body [1] and non-additive three-body potentials were generated by fitting analytical functions to the respective interaction energies computed with high-level quantum-mechanical ab initio methods. The second and third pressure virial coefficients including quantum corrections were calculated by standard numerical integration, whereas the higher virial coefficients up to seventh were determined using the Mayer-Sampling Monte-Carlo procedure [2]. In order to obtain high accuracy we developed a new formalism to incorporate non-additive three-body interactions into the third through seventh virial coefficients in a systematic and effective way [3].

Virial coefficients for argon were calculated in the range from 83 to 10,000 K. The resulting EOS was compared to the best measurements by Gilgen et al. [4] carried out with a two-sinker densimeter for pressures up to 120 bar. In the supercritical region the computed values are inside the very small error bars of the experiment (0.01%). For the critical temperature and density the calculated pressure deviates only by 1.5%, whereas for subcritical temperatures the saturated vapor pressure shows deviations of 0.1 to 0.3%. For higher pressures between 2,000 and 10,000 bar at 373 K the calculated values differ from the experimental data [5] by 1 to 9%.

References
  1. B. Jäger, R. Hellmann, E. Bich, E. Vogel, Mol. Phys. 107, 2181 (2009)

  2. J. K. Singh, D. A. Kofke, Phys. Rev. Lett. 92, 220601 (2004)

  3. R. Hellmann, E. Bich, in preparation

  4. R. Gilgen, R. Kleinrahm, W. Wagner, J. Chem. Thermodyn. 26, 383 and 399 (1994)

  5. S. L. Robertson, S. E. Babb, Jr., G. J. Scott, J. Chem. Phys. 50, 2160 (1969)

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