The inversion of thermophysical properties

Abstract

This paper applies recent developments in the understanding of thermophysical property data inversion methods, previously tested only for spherical and simplified model non-spherical systems, to a polyatomic system interacting with a realistic anisotropic potential function. The iterative inversion procedure, which generates a spherical potential energy function directly from measurements of the temperature dependence of thermophysical properties, has been applied to simulated interaction viscosities, binary diffusion coefficients and second virial coefficients corresponding to a highly anisotropic atom-diatom potential surface. The potentials, U ₁(r), obtained by inversion of the two transport properties are found to be essentially the same, in agreement with other studies involving diatom-diatom potential surfaces of somewhat lower anisotropy. By contrast, the potential from inversion of second virial coefficients is substantially different from these. The relationship of all the U ₁(r) to the full potential surface has been investigated in detail. The recently derived ‘locus-average’ potential reproduces the inversion potential quantitatively in the repulsive and long-range attractive regions, but is too shallow in the well region. Nevertheless it is shown to be significantly superior to four other averages of the full potential (spherical, Boltzmann, free energy and median) at all separations. In particular, the results cast doubt on the generality of the equivalence of U ₁(r) and the median potential, suggested by earlier studies.