R12-calibrated H2O–H2 interaction: Full dimensional and vibrationally averaged potential energy surfaces

The potential energy surface of H2O–H2 is of great importance for quantum chemistry as a test case for H2O-molecule interactions. It is also required for a detailed understanding of important astrophysical processes, namely, the collisional excitation of water, including the pumping of water masers and the formation of molecular hydrogen on icy interstellar dust grains. We have calculated the interaction for H2O–H2 by performing both rigid-rotor (five-dimensional) and non-rigid-rotor (nine-dimensional) calculations using the coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)] with moderately large but thoroughly selected basis set. The resulting surface was further calibrated using high precision explicitly correlated CCSD(T)-R12 calculations on a subset of the rigid-rotor intermolecular geometries. The vibrationally averaged potential is presented in some details and is compared with the most recent rigid-rotor calculations. We explain...