CBS extrapolation
In this section we discuss the performance of the electronic structure methods taking as reference the extrapolated and corrected Gibbs free energy and enthalpy for all the reactions in gas phase and in the solvents as well as for the activation energies (when it exists). Varandas et al 23 reported that MP2/CBS theory tends to overestimate barrier heights while M06-2X underestimate them in isomerization processes.
Figure 6 shows the difference for the Gibbs free energy obtained with the CBS extrapolation procedure and the MP2/aug-cc-pvtz, M06-2X/aug-cc-pvtz and B3LYP-D3/aug-cc-pvtz methods. While the MP2/aug-cc-pvtz results show an almost linear correlation with the extrapolated results, both DFT methods diverge significantly. The MP2/aug-cc-pvtz, M06-2X/aug-cc-pvtz and B3LYP-D3/aug-cc-pvtz results differ from the extrapolated ones by \(-0.99\ \pm 0.28\) kcal mol-1, \(2.56\ \pm 1.00\) kcal mol-1 and \(-1.05\ \pm 1.27\) kcal mol-1 (on average), respectively. Among the DFT methods, the B3LYP-D3 is closer to the CBS results, with most DFT values more positive than the CBS ones. The differences between the M06-2X/aug-cc-pvtz and the CBS values are even larger, although in this case with the DFT values more negative than the CBS ones. These differences may be observed in Figure 6.