Instability of Vacancy Clusters in Solid 4He

The behavior of pairs and clusters of vacancies in solid ⁴He crystals is studied with diffusion Monte Carlo method. We use a trial function suitable for describing solid ⁴He with long range order, arbitrary number of unoccupied sites and explicit Bose symmetry. It is found that vacancy clusters are unstable and collapse in all considered systems. We find no signature of stability of bound vacancy clusters of any size. Vacancies are found to exhibit weak attraction and a rough estimation of the binding energy is reported.