The high pressure limits for the rate constants of reactions NHT10 and NHT19 are shown in Figure , while the corresponding Arrhenius-Kooij parameters are given in Table . Both reactions are characterized by quite high energy barriers, and their rate constants follow the Arrhenius equation in the medium-to high-temperature range. Reaction NHT10 is the prototypical S N 2 reaction, which shows very large environmental effects, 84 so that accurate computations of gas-phase rate constants are the mandatory prerequisite for disentangling intrinsic and environmental effects. The value of the Best energy barrier (9.0 kJ mol -1 ) coincides with that obtained from the very accurate focal point ap-proach (FPA) given in ref. 85. Since tunneling is expected to play a negligible role, accurate rate constants should be obtained at this level. It is then remarkable that Best and junChS rate constants are very close in the whole range of temperatures, whereas the CBS-QB3 model underestimates significantly the rate constant at low temperatures. In fact, the not too high activation energy leads to significant deviation from Arrhenius behaviour at low temperatures, with the consequent non negligible impact of even relatively small errors. The isomerization of HCN to HNC (NHT19) is of paramount relevance in astrochemistry because the ratio between the two species changes in different environments. While the energy barrier is too high to allow effective isomerization in the interstellar medium (ISM), the rate constant of the reaction represents the reference value for studying catalytic effects by water molecules on icy grains. In this case, the rate constants computed by different models are in good agreement in the whole temperature range among themselves and with previous computations. The last reaction considered (NHT2) is the simplest heavy atom transfer. Figure shows that also in this case, the rate constants provided by the different methods are in good agreement for medium-to high-temperatures, whereas at low temperatures the CBS-QB3 energy barriers lead to too low rates in comparison with the (close) values of junChS-F12