Uses the PM6-DH+ procedure of M. Korth, "Third-Generation Hydrogen-Bonding Corrections for Semiempirical QM Methods and Force Fields," J. Chem. Theory Comput., 2010, 6 (12), pp 3808–3816.
Abstract: Computational modeling of biological systems is a rapidly evolving field that calls for methods that are able to allow for extensive sampling with systems consisting of thousands of atoms. Semiempirical quantum chemical (SE) methods are a promising tool to aid with this, but the rather bad performance of standard SE methods for non-covalent interactions is clearly a limiting factor. Enhancing SE methods with empirical corrections for dispersion and hydrogen-bonding interactions was found to be a big improvement, but for the hydrogen-bonding corrections the drawback of breaking down in the case of substantial changes to the hydrogen bond, e.g., proton transfer, posed a serious limitation for its general applicability. This work presents a further improved hydrogen bonding correction that can be generally included in parameter fitting procedures, as it does not suffer from the conceptual flaws of previous approaches: hydrogen bonds are now treated as an interaction term between electronegative acceptor and donor atoms, “weighted” by a function of the position of H atoms between them, and multiplied with a damping function to correct the short- and long-range behavior. The performance of the new approach is evaluated for PM6, AM1, OM3, and SCC-DFTB as well as several force-field (FF) methods for a number of standard benchmark sets with hydrogen-bonded systems. The new approach is found to reach the same accuracy as the second-generation hydrogen-bonding correction with less parameters, while it avoids among other issues the conceptual problem with electronic structure changes. SE methods augmented this way reach the accuracy of DFT-D approaches for a large number of cases investigated, while still being about 3 orders of magnitude faster. Moreover, the new correction scheme is transferable also to FF methods that were shown to have serious problems with hydrogen-bonding interactions.
The PM6-DH+ method was parameterized to reproduce interaction energies for geometries obtained from high-level quantum mechanical calculations. See accuracy.
The PM6-DH+ procedure corrects binding errors in the PM6 method. It can be used with geometry optimization or with a single point (1SCF) calculation. Normally, two or three calculations would be needed to get the binding energy.
Frozen geometries were used during the development of PM6-DH+. By contrast, in the proposed strategy, fully optimized geometries are used. No inconsistency is involved - by sketching a simple Born cycle, it becomes apparent that any errors arising from optimizing the PM6-DH+ parameters using frozen geometries and using those same parameters when calculating the binding energy using the above strategy would be very small; they would contribute only second order perturbation effects, and would be completely negligible.
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