Excitation of the lowest rotational levels of oH₂D⁺, pH₂D⁺, oD₂H⁺, and pD₂H⁺ as functions of the gas density at T = 15 K, assuming a fractional abundance X = 10^-10 for each species. The curves show the ratios of the populations to the values expected from the Boltzmann distribution. The dashed horizontal lines indicates the limit where a level is considered to be significantly populated and taken into account in the calculation of the species-to-species rate coefficients.