Ab Initio Thermophysical Properties of Gases for Applications In Metrology

Ab Initio Thermophysical Properties of Gases for Applications in Metrology

Allan H. Harvey

Thermophysical Properties Division

National Institute of Standards and Technology

325 Broadway, Boulder, Colorado, 80305

aharvey@boulder.nist.gov

ABSTRACT

Many measurements for temperature, pressure, and other thermophysical properties ultimately rely on our ability to make an absolute measurement on a simple gaseous system.  In other cases, such properties are needed to compute the corrections needed for proper interpretation of high-accuracy measurements.  The monatomic gases helium and argon are of particular importance in these contexts.

Ab initio quantum chemistry has advanced to the point where pair potentials for these gases (especially helium) can be computed with very high accuracy.  This allows the calculation of key thermophysical properties, such as the second density and acoustic virial coefficients and the low-density limit of the viscosity and thermal conductivity, with uncertainties that are in most cases smaller than can be obtained by experimental measurements.  Ab initio three-body potentials can be incorporated to allow prediction of third virial coefficients.  In some cases, this approach allows difficult experiments to be replaced by simpler measurements on systems whose properties can be calculated; in other cases, the uncertainty of fundamental measurements can be significantly reduced.

We review the state-of-the-art for first-principles calculation of thermophysical properties for helium and argon.  Particular attention is given to two recent developments:
1) Use of a path-integral Monte Carlo approach, with incorporation of quantum spin statistics, to calculate the third virial coefficient C(T) of helium at temperatures of 2.6 K and above.  The resulting uncertainties in C(T) are at least an order of magnitude smaller than those obtained from experiment.

2) Use of selected high-accuracy experimental data to “tune” an ab initio pair potential for argon to be used for accurate prediction of other thermophysical properties.

Many colleagues and collaborators have contributed substantially to the work reported here, including G. Garberoglio (Fondazione Bruno Kessler, Italy), J.B. Mehl, M.R. Moldover (NIST), and K. Szalewicz and coworkers (U. of Delaware).