  Recent advances regarding the interplay between ab initio calculations and
metrology are reviewed, with particular emphasis on gas-based techniques used
for temperature and pressure measurements. Since roughly 2010, several
thermophysical quantities - in particular, virial and transport coefficients -
can be computed from first principles without uncontrolled approximations and
with rigorously propagated uncertainties. In the case of helium, computational
results have accuracies that exceed the best experimental data by at least one
order of magnitude and are suitable to be used in primary metrology. The
availability of ab initio virial and transport coefficients contributed to the
recent SI definition of temperature by facilitating measurements of the
Boltzmann constant with unprecedented accuracy. Presently, they enable the
development of primary standards of temperature in the range 2.5-552 K and
pressure up to 7 MPa using acoustic gas thermometry, dielectric constant gas
thermometry, and refractive index gas thermometry. These approaches will be
reviewed, highlighting the effect of first-principles data on their accuracy.
The recent advances in electronic structure calculations that enabled highly
accurate solutions for the many-body interaction potentials and
polarizabilities of atoms - particularly helium - will be described, together
with the subsequent computational methods, most often based on quantum
statistical mechanics and its path-integral formulation, that provide
thermophysical properties and their uncertainties. Similar approaches for
molecular systems, and their applications, are briefly discussed. Current
limitations and expected future lines of research are assessed.
