The availability of collisional rate coefficients is a prerequisite for an
accurate interpretation of astrophysical observations, since the observed media
often harbour densities where molecules are populated under non--LTE
conditions. In the current study, we present calculations of rate coefficients
suitable to describe the various spin isomers of multiply deuterated ammonia,
namely the ND2H and ND3 isotopologues. These calculations are based on
the most accurate NH3--H2 potential energy surface available, which has
been modified to describe the geometrical changes induced by the nuclear
substitutions. The dynamical calculations are performed within the
close--coupling formalism and are carried out in order to provide rate
coefficients up to a temperature of T = 50K. For the various
isotopologues/symmetries, we provide rate coefficients for the energy levels
below ∼ 100 cm−1. Subsequently, these new rate coefficients are used
in astrophysical models aimed at reproducing the NH2D, ND2H and ND3
observations previously reported towards the prestellar cores B1b and 16293E.
We thus update the estimates of the corresponding column densities and find a
reasonable agreement with the previous models. In particular, the
ortho--to--para ratios of NH2D and NHD2 are found to be consistent with
the statistical ratios
