This material presented here is based on the work of Diedenhofen and Klamt of COSMOlogic GmbH&Co.KG.
Statistical Analysis of 779 Conformers of Organic Compounds
|PM6(Opt)||PM6 (SP)||AM1 (Opt)||AM1 (SP)|
|Mean Unsigned Errors||2.35||2.23||1.95||2.76|
The reference values of the relative energies of the conformers were calculated using the DFT method BP/TZVP*. For each conformer, two semiempirical calculations were done for each of the two semiempirical methods. In the first, shown in the table as "SP", a single point calculation was done at the optimized BP/TZVP geometry, in the second, indicated by "Opt", the semiempirical structure was optimized.
Beckes hybrid exchange + Perdew-Wang correlation
The AM1 energies changed significantly on going from the ab-initio geometry (presumably accurate) to the optimized AM1 geometry. The inference may be made that the AM1 optimized geometry differs significantly from the correct structure.
The PM6 energies change only slightly, suggesting that the PM6 optimized structures are similar to the correct structures.
*: Beckes hybrid exchange + Perdew-Wang correlation with triple zeta plus polarization basis set.