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.