  The energy transfer among the components in a gas determines its fate.
Especially at low temperatures, inelastic collisions drive the cooling and the
heating mechanisms. In the early Universe as well as in zero- or low-
metallicity environments the major contribution comes from the collisions among
atomic and molecular hydrogen, also in its deuterated version. The present work
shows some updated calculations of the H$_2$ cooling function based on novel
collisional data which explicitely take into account the reactive pathway at
low temperatures. Deviations from previous calculations are discussed and a
multivariate data analysis is performed to provide a fit depending on both the
gas temperature and the density of the gas.
