PM6-DH2 gives much improved interaction energies compared to PM6. Whereas in PM6, the average unsigned error for interaction energies for the complexes in the S22 set is 3.27 Kcal/mol, for PM6-DH2, the equivalent average unsigned error is 0.37 Kcal/mol. For details of PM6-DH2, see: M. Korth, M. Pitonak, J. Rezac, and P. Hobza, "A Transferable H-bonding Correction For Semiempirical Quantum-Chemical Methods", J. Chem. Theory Comp. 2010, 6, 344-352
Some representative complexes are shown in the Table.
Table of Representative Complexes
|Formic acid dimer||-18.61||-11.14||7.47||-18.65||-0.04|
|Lys+ - Glu-||-108.40||-98.83||9.57||-104.63||+3.77|
|Arg+ - Asp-||-110.80||-100.83||9.97||-109.80||+1.01|
Calculations were done using the
high-quality geometries from the
S22 database, see: Jurecka, P.; Sponer, J.;
Cerny, J.; Hobza, P. Phys. Chem.
Chem. Phys. 2006, 8
Reference interaction energies (CCSD(T) from the [CCSD(T)/complete basis set (CBS) limit] of the dispersion-bonded complexes.
A. PM6-DH2 Formic acid dimer
B. PM6-DH2 two separated formic acid molecules ΔHf = -169.504
Interaction energy = B - A = -18.654
Note: Heats of formation predicted by PM6-DH2 should not be compared to reference heats of formation. This is because PM6-DH2 is designed for predicting energies of interaction. If heats of formation are wanted, use PM6, for which the predicted ΔHf can be compared to experimental values.
See also PM6-DH+ accuracy.