A mixed quantum-classical (MQC) approach is developed to study energy transfer between H2O and Gly-Ala dipeptide. In this approach, the translational motion of H2O is treated classically while the rotational motion of water and the internal torsion of the dipeptide are treated quantum mechanically. This treatment results in a mixed four-dimensional quantum and three-dimensional classical treatment. By employing the AMBER force field, the MQC approach is carried out to study rotational excitation of water and torsional excitation of the dipeptide in collision between H2O and the dipeptide. For excitation of C-alpha-C and C-alpha-N torsions, the collisions are direct. In excitation of side chain torsion, the complex collision occurs with the water being trapped near the Gla-Ala. In both direct and resonant collisions, the rotational motion of water is found to be strongly excited. This MQC approach can be extended to study collision energy transfer between rigid small molecules and peptides/protein, polymer or surface. Such theoretical studies should be very useful in understanding detailed energy transfer mechanism involving biological molecules. (C) 2003 American Institute of Physics.