Browsing by Author "Jankiewicz, Wojciech"

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Correcting long-range electrostatics in DFTB
(2019-06-21)
Podeszwa, Rafal
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Jankiewicz, Wojciech
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Krzus, Magdalena
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Witek, Henryk A.
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交大名義發表
We demonstrate that the atom-based charge model implemented in the current versions of the density functional tight binding (DFTB) method fails to reproduce the correct charge distribution of a range of systems, including homonuclear molecules, graphene, and nanotubes, resulting in serious distortions in the electrostatic interactions for such systems caused by the missing quadrupole moments. In particular, this failure seriously impacts the long- and medium-range interaction energies of the DFTB plus dispersion (DFTB-D) model, leading to incorrect predictions of translational or rotational barriers in such systems. We show explicitly on examples of H-2 and N-2 that correct quadrupole momentsand consequently correct electrostatic interactionscan be restored in such systems by adding additional bond (ghost) sites to the homonuclear molecules. Attempts to determine the point charges associated with the additional sites using the usual Mulliken population analysis lead to unphysical results. Instead, these charges can be determined using the actual DFTB densities used in the parameterization process. For homonuclear molecules, we propose an extension to the DFTB-D model by adding charges that reproduce the physically correct quadrupolar charge distribution. The resulting DFTB-D-Q model greatly improves the rotational barriers for interactions of molecular hydrogen and nitrogen with benzene.
Dispersion-Corrected DFT Struggles with Predicting Three-Body Interaction Energies
(2018-10-01)
Jankiewicz, Wojciech
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Podeszwa, Rafal
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Witek, Henryk A.
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應用化學系
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Department of Applied Chemistry
We demonstrate that the dispersion-corrected density functional theory (DFT-D) schemes fall short of predicting reliable three-body interaction energies. This concerns also a popular variant of DFT-D called the "many-body dispersion" (MBD) method, which might seem surprising in the light of the fact that its name contains the very phrase "many-body". The main reason for the inaccuracy of the three-body interaction energies in the DFT-D schemes can be attributed to internal deficiencies of the standard DFT functionals that the existing "-D" methods are incapable of correcting since the main problems emerge from the terms not related to the dispersion component. At present, it seems that none of the a posteriori dispersion techniques are able to predict accurately the total interaction energy for a supermolecular system together with its simultaneous decomposition into the many-body components. On the other hand, if one is interested only in the three-body interaction energies, we propose an adjustment to the MBD approach that achieves good accuracy in conjunction with the supermolecular MP2.