To obtain the energy gradient of a specific region in a large molecule, we defined a partial energy gradient (PEG) based on the fragment molecular orbital (FMO) method. The suitability of PEG was examined by performing geometry optimizations of model systems, and we found that optimization with PEG could provide almost the same geometry obtained with the full FMO energy gradient. We also carried out calculations on real biomolecular systems; these calculations demonstrated a great advantage of PEG in computational effort. (C) 2010 Elsevier B.V. All rights reserved.