  Microlensing events observed in the line of sight toward the LMC indicate
that a significant fraction of the mass of the dark halo of the Galaxy is
probably composed of white dwarfs. In addition, white dwarf sequences have now
be observed in the HR diagrams of several globular clusters. Because of the
unavailability of white dwarf atmospheres for Teff < 4000K, cooling time scales
for white dwarfs older than ~ 10 Gyr are very uncertain. Moreover, the
identification of a MACHO white dwarf population by direct observation depends
on a knowledge of the colors and bolometric corrections of very-cool white
dwarfs.
  In this paper we present the first detailed model atmospheres and spectra of
very cool hydrogen white dwarfs for Teff < 4000K. We include the latest
description of the opacities of hydrogen and significantly, we introduce a
non-ideal equation of state in the atmosphere calculation. We find that due to
strong absorption from H_2 in the infrared, very old white dwarfs are brightest
in the V, R, and I bands, and we confirm that they become bluer in most color
indices as they cool below Teff ~ 3500K.
