To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and Moller-Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H2O)(n) and X-(H3O)(+)(H2O)(n-1) are calculated. The dissociated structures of HF and HCl can be formed for ngreater than or equal to4, while those of HBr and HI can be formed for ngreater than or equal to3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for ngreater than or equal to4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X-H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules. (C) 2004 American Institute of Physics.