The energy density analysis (EDA) under the existence of an external field is presented and applied to the embedded cluster models of an MgO ionic crystal. It is clarified that the cohesive energy of an ionic crystal can be evaluated by the EDA technique. The effects of different sizes and strengths of external fields are investigated by the use of the calculated cohesive energy, and the homogeneity of the model cluster is evaluated by calculating the cohesive energies at different sites. (c) 2005 Elsevier B.V. All rights reserved.