  We present quantum dynamical calculations that describe the rotational
excitation of H$_2$O due to collisions with H atoms. We used a recent, high
accuracy potential energy surface, and solved the collisional dynamics with the
close-coupling formalism, for total energies up to 12 000 cm$^{-1}$. From these
calculations, we obtained collisional rate coefficients for the first 45 energy
levels of both ortho- and para-H$_2$O and for temperatures in the range T =
5-1500 K. These rate coefficients are subsequently compared to the values
previously published for the H$_2$O / He and H$_2$O / H$_2$ collisional
systems. It is shown that no simple relation exists between the three systems
and that specific calculations are thus mandatory.

Astrophysics of Galaxies;Chemical Physics