Comparison of HD abundance considering the different hypothesis (state-to-state, case (1) and case (2)). (a) Deuterium molecular fraction as a function of distance from the ionization front, d. The red line is for case (2), all internal energy used, the green line is for the state-to-state chemistry, and blue is for case (1) analytic expressions from Gay et al. (2011) using no internal energy. (b) Abundance of level v = 1 and J = 2.