Potential energy surfaces of antiparallel water-water interactions

Antiparallel water-water interaction is a significant interaction between two water molecules and plays an important role in liquid water. The potential energy surface of antiparallel water-water interaction was calculated at accurate CCSD(T)/CBS level of theory by systematic changes of the torsion angle T-HOHO, parallel displacement r, normal distance R, and water-water dihedral angle Pa/P-b. The results show that the most stable geometry of antiparallel water-water interaction has an interaction energy of -4.22 kcal/mol and T-HOHO = 140 degrees. A significant portion of the antiparallel interactions have interaction energies more negative than 2.0 kcal/mol. The angle a of the most stable geometries of antiparallel water-water interactions is in the range 110. -120.. Comparison with classical hydrogen bonds in water dimers shows that hydrogen bonds with values of angle alpha < 140 degrees have interaction energies up to 3.2 kcal/mol, while a significant number of antiparallel water-water interactions is stronger than it. The antiparallel geometry is a low barrier transition state between two hydrogen bonded minima with changed acceptor-donor roles of two water molecules.