We are reporting density functional theory results for the binding energies, structures, and vibrational spectra of (H-Cl)(2-6) and (H-F)(2-10) clusters. The performance of different functionals has been investigated. The properties of HF clusters predicted by hybrid functionals are in good agreement with experimental information. The HCl dimer binding energy DeltaE(e) is underestimated by hybrid functionals. The Perdew and Wang exchange and correlation functional (PW91) result for DeltaE(e) is -9.6 kJ mol(-1), in very good agreement with experiment (-9.5 kJ mol(-1)). However, PW91 overestimates binding energies of larger clusters. Hydrogen bonding cooperativity depends on the cluster size n but reaches a limit for moderately sized clusters (n=8 for HF). The average shift to low frequencies (Deltanu) of the X-H (X=Cl,F) stretching vibration relative to the monomer is in good agreement with experimental data for HF clusters in solid neon. However, some discrepancies with experimental results for HCl clusters were observed. The behavior of Deltanu as a function of the cluster size provides an interesting illustration of hydrogen-bond cooperative effects on the vibrational spectrum. The representation of the electronic density difference shows the rearrangement of the electronic density induced by hydrogen bonding in the clusters and supports the view that hydrogen-bond cooperativity is related to electronic sharing and delocalization. (C) 2003 American Institute of Physics.