Divide-and-Conquer-Type Density-Functional Tight-Binding Simulations of Hydroxide Ion Diffusion in Bulk Water

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Understanding ion diffusion in water is crucial for chemical reactions and biological processes.
• The hydroxide ion (OH-) plays a key role in many aqueous systems.
• Accurate simulation of ion transport requires robust computational methods.

Purpose of the Study:

• To investigate the diffusion mechanism of the hydroxide ion in bulk water.
• To validate simulation methods against experimental data.
• To elucidate the role of hypercoordination in ion diffusion.

Main Methods:

• Linear-scaling divide-and-conquer density-functional tight-binding molecular dynamics (DC-DFTB-MD) simulations.
• Utilized unit cells of varying sizes (522, 1050, 4999 water molecules).
• Improved oxygen-oxygen pair potential via iterative Boltzmann inversion, referencing DFT-MD.

Main Results:

• Calculated diffusion coefficients and Arrhenius diffusion barriers showed good agreement with experimental values.
• Analysis of coordination number distribution supported the proposed diffusion mechanism.
• Potential of mean force calculations indicated a dynamical hypercoordination diffusion pathway.

Conclusions:

• The study confirms a dynamical hypercoordination diffusion mechanism for hydroxide ions in water.
• DC-DFTB-MD simulations, with improved potentials, accurately reproduce experimental diffusion behavior.
• This work provides valuable insights into ion transport phenomena in aqueous solutions.