An isolated cluster method has been implemented to avoid the finite size error in Quantum Monte Carlo (QMC). With this method, the dissociation energy of solid can be obtained by extrapolating several finite size clusters into infinite. As an example, the dissociation energy of MgH2 is predicted, and the result shows that there is only 0.6 kJ/mol difference from the experimental results. This method can be a benchmark simulation of MgH2 solid. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.