A STRUCTURAL AND THERMODYNAMIC STUDY.Stochastic exploration of the potential energy surface of (ethanol) 4 -water heteropentamers through simulated annealing calculations was used to find probable structures of these clusters.Subsequent geometry optimization with the B3LYP/6-31+G(d) approach of these initial structures led to 13 stable heteropentamers.The strength of the hydrogen bonds of the type O-H---O (primary) and their spatial arrangements seem to be responsible for the geometric preferences and the high stability of these heteropentamers.This result is a consequence of the presence of the cooperative effects among such interactions.There is no significant influence of the secondary hydrogen bonds (C-H---O) on the stability of the heteropentamers.