Density functional theory and ab initio calculations were performed to elucidate the hydrogen interactions in (H 2 O 4 ) n (n 5 1-4) clusters. The optimized geometries, binding energies, and harmonic vibrational frequencies were predicted at various levels of theory. The trans conformer of the H 2 O 4 monomer was predicted to be the most stable structure at the CCSD(T)/aug-cc-pVTZ level of theory. The binding energies per H 2 O 4 monomer increased in absolute value by 9.0, 10.1, and 11.8 kcal/mol from n 5 2 to n 5 4 at the MP2/cc-pVTZ level of theory (after the zero-point vibrational energy and basis set superposition error corrections). This result implies that the intermolecular hydrogen bonds were stronger in the long-chain clusters, that is, the formation of the longer chain in the (H 2 O 4 ) n clusters was more energetically favorable.