In this section, we analyze the performance of three composite schemes (junChS-F12, Best The reaction of OH with CH 4 (HT4) is very important in the Earth's troposphere since it accounts for about 90% of the total CH 4 sink. The junChS-F12 and Best energy barriers are quite close (27.7 and 28.1 kJ mol -1 , respectively), whereas a slightly lower value (26.0 kJ mol -1 at the W1 level ) was estimated in the most exhaustive computation of the rate constant performed till now. In any case, the reaction shows a strongly non-Arrhenius behavior (related also to the presence of a hindered rotation at the transition state) and the agreement between different model chemistries is only fair at low temperatures (see panel (a) of Figure ). While a more comprehensive analysis of all the factors (path curvature, anharmonicity, etc.) playing a role in determining the rate constant of this reaction is beyond the scope of the present paper, we point out that the difference between harmonic and anharmonic ZPEs is not negligible (about 1 kJ mol -1 for both the forward and backward reaction).