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
| Complex | CCSD(T)/CBS | PM6 | Error | PM6-DH2 | Error |
| A-T WC | -16.37 | -9.06 | 7.31 | -16.45 | -0.08 |
| G-C WC | -31.40 | -21.35 | 10.05 | -30.75 | +0.65 |
| A-T Stack | -12.23 | -4.94 | 7.39 | -11.67 | +0.54 |
| G-C Stack | -19.02 | -13.07 | 5.93 | -19.66 | -0.64 |
| Water dimer | -5.02 | -3.94 | 1.08 | -4.90 | +0.12 |
| Formic acid dimer | -18.61 | -11.14 | 7.47 | -18.65 | -0.04 |
| Formamide dimer | -15.96 | -12.55 | 3.41 | -15.87 | +0.10 |
| 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
(17), 1985-1993
Reference interaction energies (CCSD(T) from the [CCSD(T)/complete basis
set (CBS) limit] of the dispersion-bonded complexes.
Worked example:
A. PM6-DH2 Formic acid dimer
ΔHf
= -188.158
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.