Abstract
Equation of state of $^{4}\text{H}\text{e}$ 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 are 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 $10.5\ifmmode\pm\else\textpm\fi{}1.2\text{ }\text{K}$. 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 $^{4}\text{H}\text{e}$ was observed in a recent experiment.
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