Abstract: Water clusters play a major role in infrared absorption in the atmosphere. The role of water clusters in the radiative balance of the atmosphere requires information on some absorption features, especially the normal modes of vibrations. In this research, we studied water clusters using density functional theory. The performance of 66 functionals, in combination with 6 basis sets, was tested on water monomer using Amsterdam Density Functional (ADF) quantum chemistry package. Results were compared to the experimental values of the vibrational frequencies and geometric properties of water monomer. The comparison suggests that GGA (generalized gradient approximations)/ BLYP-D(BJ) functional and ET-pVQZ basis set is the best combination for calculating the vibrational frequencies for the monomer. The vibrational frequencies and IR intensities for clusters n = 2 – 5, where n is the number of monomers constituting the cluster were obtained using the above combination of functional and basis set. We obtained vibrational spectrum which may provide targets for experiments. The spectra of these water clusters, n = 2 – 5, show a broadened band profiles compared with monomers. In this way, water clusters may play a role in the anomalous solar absorption

Key Words: water cluster, density functional theory, computational chemistry