The resolution of the identity second-order Moller-Plesset perturbation theory (RI-MP2) was introduced into the fragment molecular orbital (FMO) method, where the program of the RI-MP2 method was newly developed. After some test calculations with a small peptide, the performance of the RI-MP2 method with the FMO scheme was demonstrated for two biomolecular systems: the human immunodeficiency virus type 1 protease with the lopinavir molecule, and the prion protein with the GN8 molecule. These calculations showed the great advantage of FMO calculations using the RI-MP2 method over ordinary FMO calculations. (C) 2009 Elsevier B. V. All rights reserved.