Monte Carlo study of vacancies in the bcc and hcp phases of He-4

We use the shadow wave function formalism to determine the energy of format ion of single and double vacancies in He-4 crystals at T=0 K. Data are pres ented for both the bcc and hcp phases. The activation energy for a single v acancy in bcc He-4 was found to be about 50% of that in hcp 4He, which is a pproximately 15.6 K. By determining the occupation of the Voronoi regions a round the crystal sites, we determine the location of vacancies in the crys tal and studied the relaxation of the neighboring atoms. We also present da ta on the correlations between vacancies, and between vacancies and He-3 im purities. Following the position of the vacancy through successive configur ations we observed the motion of the vacancy as seen in our Monte Carlo sim ulations. On the shorter Monte Carlo time scales, greater vacancy motion wa s observed in the bcc phase than in the hcp phase. [S0163-1829(99)07229-X].