The important outcome of this comparison is that although having a level specific HO-RR model does not have a large effect on thermochemistry, it is still important to consider the low lying electronic states of the molecule since they get populated at these high temperatures thus affecting the thermochemistry. In the case of C 2 , the difference is large and up to 12% when going from the ground state to including low-lying excited states, although for case (ii) and (iii) the differences are insignificant (up to 2%). A similar comparison for C 3 and C 4 , shown in Fig. and, leads to the observation that the differences between case (ii) and case (iii) are also important particularly at higher temperatures as the excited states get more populated. These differences are up to 23%. In the case of C 4 , due to the large size of the molecule, slightly non-linear optimum geometries of the excited states and the low-lying first excited electronic state, these differences are accentuated even at lower temperatures. Figure shows a comparison of the enthalpies for the three molecules. As can be observed, the effects on the enthalpy are rather negligible.