Quasi-classical rate coefficient calculations for the rotational (de) excitation of H2O by H-2

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 H2O 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 H2O 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 H2O-H-2 potential energy surface using quasi- classical trajectory calculations. Trajectories are sampled by a canonical Monte- Carlo procedure. H2 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.