Ab initio calculations an performed on the hydrogen bond interaction between HCN and water to analyze the structure, binding energy and change in vibrational frequencies of the HCN:H2O isomer. After geometry optimization, single-point calculations are made with many-body perturbation/coupled-cluster theories with different basis sets. At the highest level, CCSD(T), we find that the binding energy between HCN and water is 3.4 kcal/mol, after correcting for the basis-set-super position error. Changes in intra-molecular vibrational frequencies are analyzed.