The ionic dissociation of H-X acids (X=F, Cl, Br, I) in water was examined by conducting a theoretical study on the properties of the clusters formed by the acids with up to five water molecules: X-H(H2O)(n) (n=1-5). Calculations were done using the DFT/B3LYP and MP2 methods in conjunction with the TZVP basis set and allowed the identification of several minima on the potential surfaces for the clusters. Based on the results, the MP2 method predicts a lower tendency to ionization than does the DFT/B3LYP method; however, both methods provide similar results. The dissociation characteristics of the acids were examined in terms of various parameters including the lengths of the bonds involved in the proton transfer and the frequencies associated with the X-H and O-H stretching modes in the bonds taking part in the proton transfer. The successive incorporation of water molecules to the cluster was found to lengthen X-H distances and simultaneously decrease O...H distances. In addition, the X-H stretching frequency underwent a marked redshift; the signal disappeared in the ionized structures, at the expense of a new series of bands around 2800 cm(-1) due to stretching vibrations of the O-H bond in the H3O+ ion. Hydrogen fluoride failed to dissociate in the clusters considered; in fact, while some structures were ionized, they were not the most stable configurations for the corresponding clusters. In HCl and HBr, the dissociated structure was the most stable in the clusters of four or more water molecules (n=4-5); however, HBr exhibited a stronger tendency to dissociating above n=3. Finally, HI exhibited dissociation at n>2.(C) 2002 American Institute of Physics.