In this work, we present the H2O interaction with H2O, (H2O)(2) and CO molecules using ab initio calculation combined with both supermolecular and multipolar expansion methods. Molecular structures and electronic properties of monomers were calculated with MP2/6-311++G(3d,3p) level. The interaction energy for the H2O dimer, trimer and H2O... CO complex are -4.88, -13.85 and -1.68 kcal/mol, respectively, using the supermolecular approach and -4.667, -13.858 and -1.714 kcal/mol, respectively, using the multipolar expansion method, where the main contribution is the electrostatic one with 81.4%, 56.6% and 41.7% for the H2O dimer, trimer and H2O-CO, respectively, showing that these polar molecules interact mainly through electrostatic forces. (c) 2006 Elsevier B.V. All rights reserved.