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Ab initio quantum chemical studies at the HF and MP2 levels with the 6-31G** basis set were performed for hydrogen-bonded complexes of hydroxyurea with a water molecule (HUW). Since at both the HF and MP2 levels of theory the keto forms of both E and Z tautomers of hydroxyurea are found to be much more stable compared to iminol forms, only keto forms were included in our study of HUW. The interaction energies were corrected for the basis set superposition error (BSSE) by using the full Boys−Bernardi counterpoise correction scheme. The zero-point vibrational energies are calculated at the MP2/6-31G**//MP2/6-31G** level. In addition, single-point calculations were performed at the MP2/6-311++G(2df,2pd)//MP2/6-31G** and at the CCSD(T)/6-31G**//MP2/6-31G** levels of theory. It was shown that the complex I of HUW is the global minimum on the potential energy surface and also has the largest interaction energy. On the basis of the results of these calculations, the stability of the different conformers of HUW, the nature of the specific hydrogen-bonding interactions, and the interaction energies are discussed.

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