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 cry stal at T=0 K. We find that this energy is a strong function pf densit y, in agreement with experiment. The use of a more efficient sampling with a smart Monte Carlo technique, allowed us to observe the motion o f the vacancy through the crystal. We also present data on the occupat ion of the Wigner-Seitz cells. The occupation of the cells changes alo ng 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 perf ect hcp and bcc crystals described by a SWF.