A theoretical study of uracil–(H2O)n, n = 2 to 4

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Abstract
The potential energy surfaces of the uracil–(H2O)n clusters with n = 2, 3 and 4 are explored with an anisotropic atom–atom potential. The structures and binding energies of the uracil–(H2O)2 structures and of selected uracil–(H2O)3 and uracil–(H2O)4 structures are recomputed with second-order Møller–Plesset (MP2) perturbation theory and the interaction-optimized DZPi basis set. Single point calculations are carried out at the optimized geometries using the slightly larger ESPB basis set. Full as well as rigid-body geometry optimizations were carried out. The use of rigid monomers causes the binding energies to be underestimated by 10% and the hydrogen-bond distances to be overestimated by about 0.1 Å. It is found that re-optimization of the hydrogen-bond lengths with counterpoise correction increases the binding energy by about 3%, and increases the hydrogen-bond lengths by approximately 0.1 Å. These results show that care must be taken to obtain unbiased results from ab initio calculations. For example, the uracil–(H2O)3 binding energy obtained from a standard, rigid-body MP2/DZPi geometry optimization is as much as 18 kJ mol−1 smaller than the binding energy obtained from a full geometry optimization including counterpoise corrections for the hydrogen-bond distances. Our best estimates for the binding energies of the uracil–(H2O)2, uracil–(H2O)3 and uracil–(H2O)4 global minima are 93.6, 132.9 and 165.8 kJ mol−1, respectively.

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