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.

The hydrogen and water molecules are ubiquitous in the Universe. Their mutual collisions drive water masers and other line emission in various astronomical environments, notably molecular clouds and star forming regions. We report here a full nine-dimensional interaction potential for H 2 O−H 2 calibrated using high-accuracy, explicitly correlated wavefunctions. All degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance. As a first application, we present rate constants for the vibrational relaxation of the v 2 bending mode of H 2 O obtained from quasi-classical trajectory calculations in the temperature range 500−4000 K. Our high temperature (T ≥ 1500 K) results are found compatible with the single experimental value at 295 K. Our rates are also significantly larger than those currently used in the astrophysical literature and will lead to a thorough re-interpretation of vibrationally excited water emission spectra from space.

Aims. Using a newly determined 5D potential energy surface for H 2 -H 2 O we provide an extended and revised set of rate coefficients for de-excitation of the lowest 10 para-and 10 ortho-rotational levels of H 2 O by collisions with para-( j = 0) and ortho-H 2 ( j = 1), for kinetic temperatures from 5 K to 20 K. Methods. Our close coupling scattering calculations involve a slightly improved set of coupled channels with respect to previous calculations. In addition, we discuss the influence of several features of this new 5D interaction on the rotational excitation cross sections. Results. The new interaction potential leads to significantly different rate coefficients for collisions with para-H 2 ( j = 0). In particular the de-excitation rate coefficient for the 1 10 to 1 01 transition is increased by up to 300% at 5 K. At 20 K this increase is 75%. Rate coefficients for collisions with ortho-H 2 ( j = 1) are modified to a lesser extent, by up to 40%. The influence of the new potential on collisions with both para-( j = 0) and ortho-H 2 ( j = 1) is expected to become less pronounced at higher temperatures.