The use of the GSAM approach for the structural investigation of low-lying isomers of molecular clusters from density-functional-theory-based potential energy surfaces: the structures of microhydrated nucleic acid bases.

This study presents structural properties of microhydrated nucleic acid bases (NABs) - uracil (U), thymine (T), guanine (G), adenine (A), and cytosine (C) - investigated by theoretical computations at the B3LYP level of theory. To obtain the different representations of these microhydrated species, we applied the GSAM procedure: the most stable conformers labeled X,nH2O (X = U, T, G, A and n = 1...5) for which the Boltzmann population is higher than 2% at 298K are calculated at the B3LYP and B3LYP-D levels of theory. At the B3LYP level, our calculated geometries are compared with those obtained in the literature. New physically relevant isomers are found with the GSAM algorithm, especially for the tetra- and pentahydrated species. The use of DFT-D functional does not strongly modify the relative energies of the isomers for the monohydrated species. When the number of water molecules increases, the results become extremely sensitive to the consideration of dispersion contributions.