Bond separation energies performance

With the exception of semi-empirical models, all models provide very good descriptions of relative nitrogen basicities. Even STO-3G performs acceptably compounds are properly ordered and individual errors rarely exceed 1 -2 kcal/mol. One unexpected result is that neither Hartree-Fock nor any of the density functional models improve on moving from the 6-3IG to the 6-311+G basis set (local density models are an exception). Some individual comparisons improve, but mean absolute errors increase significantly. The reason is unclear. The best overall description is provided by MP2 models. Unlike bond separation energy comparisons (see Table 6-11), these show little sensitivity to underlying basis set and results from the MP2/6-3IG model are as good as those from the MP2/6-311+G model. 

Table 12-21 Performance of Localized MP2 Models on Energies of Bond Separation Reactions... 

The potential function V (l) has a minimum at Iq > lo, in accord with the intuitive expectation that the bond separation should increase in the presence of a tensile stress. The new activation energy (D ) required to break the stressed bond could be calculated from the principle of virtual work performed on the bond in going from Ifl to lb (Figure 15.19). 

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