Variational study of vacancies in solid He-4 with shadow wave functions

We employ the shadow wave function (SWF) formalism to obtain estimates of the energy of formation of a vacancy in hcp, fee, and bcc He-4 crystal at T=0 K. We find that this energy is a strong function pf density, in agreement with experiment. The use of a more efficient sampling with a smart Monte Carlo technique, allowed us to observe the motion of the vacancy through the crystal. We also present data on the occupation of the Wigner-Seitz cells. The occupation of the cells changes along the run, showing the mobility of the particles and of the vacancy. As a byproduct of this study we present results for the energy of perfect hcp and bcc crystals described by a SWF.