For this reaction, we chose the M08-HX functional with the MG3S basis set, 108 which provides a potential energy barrier of 6.46 kcal/mol. This value is in good agreement with The electronic density functional is M08-HX in all cases. The results labeled "standard" were calculated with the standard vibrational scale factor (0.973), and those labeled "SRP" were calculated with a specific reaction parameter for the vibrational scale factor.