The accuracy of various levels of theory in calculating the properties of hydrogen bonded systems is assessed. An adequate treatment of both electron correlation and basis set superposition error is generally required for quantitative estimates of the equilibrium intermolecular distance and interaction energy. Polarized double-ζ basis sets can provide useful information about the perturbations caused by hydrogen bond formation upon the vibrational frequencies and intensities of the individual monomers. The uniqueness of the hydrogen bond is underscored by detailed comparison with a lithium bonded complex. In particular, the calculations provide insights into the origin of the very different behavior of the v8 stretch in the two different systems. The cooperativity involved in a number of sequential hydrogen bonds is examined using (HF)3 and (HCl)3 as model systems. The internal stretching frequencies exhibit an especially large change on going from dimer to trimer.