  We present a complete calculation, fully accounting for quantum effects and
for molecular flexibility, of the first dielectric virial coefficient of water
and its isotopologues. The contribution of the electronic polarizability is
computed from a state-of-the-art intramolecular potential and polarizability
surface from the literature, and its small temperature dependence is
quantified. The dipolar polarizability is calculated in a similar manner with
an accurate literature dipole-moment surface; it differs from the classical
result both due to the different molecular geometries sampled at different
temperatures and due to the quantization of rotation. We calculate the dipolar
contribution independently from spectroscopic information in the HITRAN2020
database and find that the two methods yield consistent results. The resulting
first dielectric virial coefficient provides a complete description of the
dielectric constant at low density that can be used in humidity metrology and
as a boundary condition for new formulations for the static dielectric constant
of water and heavy water.
