(226d) 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).