Molecular dynamics studies for the new refrigerant R152a with simple model potentials

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Abstract
The new refrigerant R152a (CH3-CHF2) is modelled as a fluid of homonuclear two-centre Lennard-Jones molecules with a point dipole along the axis. This 2CLJD potential has four parameters which were determined by using results of previous and new molecular dynamics simulations for 2CLJD molecules of elongation L* = 0·505 and different reduced dipole moments μ*. For each of these dipole moments zero pressure liquid densities at two temperatures were taken to determine the Lennard-Jones parameters ϵ and σ from a fit to experimental saturated liquid densities. A subsequent comparison of calculated second virial coefficients with experimental values led to the conclusion that μ*2 = 8 in combination with ϵ/k = 119·0K and σ = 3.845 Å is a reasonable choice. Further simulations with those parameters, all performed with vectorized codes on a CYBER 205, gave reasonable predictions of the thermodynamic properties. As the reduced dipole moment μ*2 = 8 corresponds to 2·73 D compared to the experimental value of 2·27 D, the effect of rotating the dipole vector out of the molecular axis was also investigated. It is found that for a dipole vector forming some angle with the molecular axis a smaller value of the dipole moment is required to produce similar thermodynamic properties as for the case when the dipole is along the axis. Spherical harmonic expansion coefficients gu′m of the pair correlation function are given for the model with and without the dipole and also for three different orientations of the dipole vector with respect to the molecular axis.