Theoretical studies of uracil-(H2O)n (n = 1-7) clusters by ab initio and ABEEMσπ/MM fluctuating charge model

Uracil-(H2O)n (n = 1-7) clusters were systemically investigated by ab initio methods and the newly constructed ABEEMσπ/MM fluctuating charge model. Water molecules have been gradually placed in an average plane containing uracil. The geometries of 38 uracil-water complexes were obtained using B3LYP/6-311++G∗∗ level optimizations, and the energies were determined at the MP2/6-311++G∗∗ level with BSSE corrections. The ABEEMσπ/MM potential model gives reasonable properties of these clusters when comparing with the present ab initio data. For interaction energies, the root mean square deviation is 0.96 kcal/mol, and the linear coefficient reaches 0.997. Furthermore, the ABEEMσπ charges changed when H2O interacted with the uracil molecule, especially at the sites where the hydrogen bond form. These results show that the ABEEMσπ/MM model is fine giving the overall characteristic hydration properties of uracil-water systems in good agreement with the high-level ab initio calculations.