Correcting long-range electrostatics in DFTB

Area of Science:

• Computational Chemistry
• Materials Science

Background:

• The atom-based charge model in density functional tight binding (DFTB) is widely used.
• Current DFTB versions struggle to accurately represent charge distribution in systems like homonuclear molecules, graphene, and nanotubes.

Purpose of the Study:

• To address the limitations of the standard DFTB charge model.
• To improve the accuracy of electrostatic interactions and energy predictions in DFTB, particularly for long- and medium-range interactions.

Main Methods:

• Implementing additional bond (ghost) sites in homonuclear molecules to restore quadrupole moments.
• Determining site charges using DFTB densities from the parameterization process, rather than Mulliken population analysis.
• Proposing an extension to the DFTB plus dispersion (DFTB-D) model, termed DFTB-D-Q, for homonuclear molecules.

Main Results:

• The standard DFTB charge model fails to reproduce correct charge distributions and electrostatic interactions due to missing quadrupole moments.
• The proposed DFTB-D-Q model significantly enhances the accuracy of rotational barriers for interactions involving molecular hydrogen and nitrogen with benzene.
• The new method successfully restores physically correct quadrupolar charge distributions.

Conclusions:

• The DFTB charge model requires refinement to accurately capture electrostatic interactions, especially for systems with significant quadrupole moments.
• The developed DFTB-D-Q model offers a more reliable approach for calculating interaction energies and barriers in relevant molecular systems.
• Accurate charge distribution modeling is crucial for predicting system behavior in computational chemistry.