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The potential energy surface of H(2)O-H(2) is of great importance for quantum chemistry as a test case for H(2)O-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 H(2)O-H(2) 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, in particular, as to why vibrationally averaged rigid-rotor geometries are a better choice than equilibrium geometries. Our fit of the vibrationally averaged surface provides for the first time an accuracy of approximately 3 cm(-1) in the van der Waals minimum region of the interaction. The overall accuracy of the nine-dimensional surface and fit is lower but remains of the order of 3%-4% of the anisotropy in the domain spanned by the vibrational functions.

Context. The interpretation of water line emission from existing observations and future HIFI/Herschel data requires a detailed knowledge of collisional rate coefficients. Among all relevant collisional mechanisms, the rotational (de)excitation of H 2 O by H 2 molecules is the process of most interest in interstellar space. Aims. To determine rate coefficients for rotational de-excitation among the lowest 45 para and 45 ortho rotational levels of H 2 O colliding with both para and ortho-H 2 in the temperature range 20−2000 K. Methods. Rate coefficients are calculated on a recent high-accuracy H 2 O−H 2 potential energy surface using quasi-classical trajectory calculations. Trajectories are sampled by a canonical Monte-Carlo procedure. H 2 molecules are assumed to be rotationally thermalized at the kinetic temperature. Results. By comparison with quantum calculations available for low lying levels, classical rates are found to be accurate within a factor of 1−3 for the dominant transitions, that is those with rates larger than a few 10 −12 cm 3 s −1 . Large velocity gradient modelling shows that the new rates have a significant impact on emission line fluxes and that they should be adopted in any detailed population model of water in warm and hot environments.

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