Ab initio calculations on uracil-water

The potential energy surface for the interaction of uracil with one water m olecule is investigated using ab initio techniques. The structures of four cyclic minima, as well as two transition-state structures, have been determ ined using second-order Moller-Plesset perturbation theory (MP2) and the in teraction-optimized DZPi basis set. At the optimized geometries, the counte rpoise-corrected interaction energies have also been computed with a slight ly larger basis set containing bond functions, labeled ESPB. The MP2/ESPB c alculations predict D-e for the four uracil-water minima to be -40.0, -31.8 , -33.5, and -26.6 kJ/mol. The barrier height between the global minimum an d the adjoining local minimum (with D-e = -31.8 kJ/mol) is found to be as m uch as 23 kJ/mol, while the barrier height between the two most stable loca l minima (D-e = -33.5 and -31.8 kJ/mol) is only 10 kJ/mol. For the global m inimum we also investigated the effect of basis set superposition error (BS SE) on the two hydrogen bond distances, as well as the effect of freezing t he monomer geometries during optimization. It is found that BSSE decreases the hydrogen bond lengths by about 0.1 Angstrom, while freezing the intramo lecular geometries reduces the uracil-water interaction energy by less than 2 kJ/mol.