  Equation of state of He-4 hcp crystals with vacancies is determined at zero
temperature using the diffusion Monte Carlo technique, an exact ground state
zero-temperature method. This allows us to extract the formation enthalpy and
isobaric formation energy of a single vacancy in otherwise perfect helium
solid. Results were obtained for pressures up to 160 bar. The isobaric
formation energy is found to reach a minimum near 57 bar where it is equal to
$10.5\pm 1.2$ K. At the same pressure, the vacancy formation volume exhibits a
maximum and reaches the volume of the unit cell. This pressure coincides with
the pressure interval over which a peak in the supersolid fraction of He-4 was
observed in a recent experiment.
