The pre-reactive complexes formed by oxirane (CH2)(2)O and diatomic halogens (XY, X = H, F and Y = F, Cl) were investigated through ab initio calculations at the MP2 = full level using 6-311 + + G(d,p) basis functions. While in the case of oxirane and HX (X = F, Cl) rather strong hydrogen bonds form between the oxygen atom of oxirane and H atom of HX, the interaction is weaker in the case of F-2. Significant changes in monomer geometries have been observed, leading to a large low-frequency shift of the XY vibration. Energy partitioning analysis of the total interaction energy has been performed using symmetry adapted perturbation theory to probe the nature of the forces involved in the complexes and how they differ on the change of X and Y atoms. (C) 1998 Elsevier Science B.V.