Mixed hydrogen-bonded clusters H2O . . . HCN, HCN . . .H2O, H2O . . . HCN . . .H2O, and H2O . . . HCN . . . HCN . . .H2O are studied by using ab initio calculations. The optimized structures and harmonic vibrational frequencies are obtained at the DFT/B3LYP and MBPT/MP2 levels with the 6-311++G(d,p) basis set. To investigate electron correlation effects on the binding energies, single-point calculations are also performed using the CCSD(T) method with the optimized MP2 geometric,,. The complexation energies are obtained for these systems including correction for basis set superposition error. In addition, the cooperative effects in the properties of the complexes are investigated quantitatively. We found a cooperativity contribution of around 10% relative to the total interaction energy of the complex H2O . . . HCN . . .H2O. In the case of H2O . . . HCN . . . HCN . . .H2O, the binding energy of the HCN . . . HCN is ca. 8 kJ/mol stronger in the mixed tetramer than in the corresponding isolated dimer. The effects of higher-order electron correlation are found to be mild, with MP2 giving a well-balanced result. Cooperative effects are predicted either by MP2 or by B3LYP in hydrogen bond distances and dipole moments of the clusters. In contrast, the B3LYP functional fails to account fur the out-of-plane bend angle in H2O . . . HCN, which is well-described by the MP2 method.