A molecular-orbital (MO) localization scheme for peptide molecules has been proposed. Combining the minimum orbital deformation (MOD) and population localization methods, the present scheme produces MOs localized within predefined fragment regions. Because the transformed MOs become similar to externally-introduced reference MOs, we can introduce MO representations commonly used in chemistry. Calculated Lowdin populations and overlap integrals showed that the extent of localization was satisfactory. The transformed MOs were applied to the configuration interaction singles (CIS) calculations. The results clearly showed that the chemically-intuitive MO localization simplified not only one-electron orbitals, but also the structure of the excited-state wave functions. (C) 2011 Elsevier B.V. All rights reserved.