(695f) An Ab Initio Equation of State for Supercritical Helium-4 Encapsulating Quantum Effects

Using state-of-the-art ab initio potentials and novel Monte Carlo methods, a predictive virial equation of state for helium-4 has been developed that is sufficiently accurate to be used in the calibration of a densimeter above 223 K.^1-3  The temperature range is limited by application of the semiclassical approximation to the fourth and fifth virial coefficients.  Here we present calculations of these terms that fully account for the quantum contributions: the increased repulsion associated with quantum diffraction and the apparent attraction associated with the exchange interaction of helium-4 atoms.  Our approach couples path-integral formulations with the Mayer-sampling Monte Carlo method and relies upon on-the-fly decompositions of the calculation into components that balance the time requirements per Monte Carlo step and the number of steps required.  We discuss the ability of the resulting equation of state to estimate the critical point and describe the supercritical phase of helium-4.

(1) G. Garberoglio and A. H. Harvey.  J. Chem. Phys. 2011, 134, 134106.

(2) G. Garberoglio, M. R. Moldover,  and A. H. Harvey.  J. Res. Natl. Inst. Stan. 2011, 116, 729-742.

(3) K. R. S. Shaul, A. J. Schultz,  D. A. Kofke, and M. R. Moldover.  Chem. Phys. Lett. 2012, 531, 11–17.