The self-consistent charge density functional tight binding method applied to liquid water and the hydrated excess
C Mark Maupin1, Bálint Aradi, Gregory A Voth
1Center for Biophysical Modeling and Simulation and Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, USA.
The self-consistent charge density functional tight binding (SCC-DFTB) method, even with hydrogen bonding improvements, struggles to accurately model bulk water and hydrated excess protons. Results show deviations from experimental data for water properties and proton diffusion.
Area of Science:
- Computational Chemistry
- Physical Chemistry
- Theoretical Chemistry
Background:
- The self-consistent charge density functional tight binding (SCC-DFTB) method is an approximate electronic structure technique gaining traction for biological systems in water.
- Previous gas-phase studies suggest SCC-DFTB can approximate geometries, energies, and vibrational frequencies.
- Validation for bulk water properties and the hydrated excess proton remains limited.
Purpose of the Study:
- To evaluate the original SCC-DFTB and a modified HBD-SCC-DFTB method for modeling bulk water and hydrated excess proton properties.
- To compare SCC-DFTB results against experimental data and Car-Parrinello molecular dynamics (CPMD) simulations.
- To assess the structural, energetic, and dynamical accuracy of SCC-DFTB for aqueous systems.
Main Methods:
- Simulations of 128 water molecules with and without a hydrated excess proton using SCC-DFTB and HBD-SCC-DFTB.
- Comparison with experimental data and CPMD simulations employing the HCTH/120 functional.
- Periodic boundary conditions with Ewald summation for long-range electrostatics were utilized.
Main Results:
- Original SCC-DFTB poorly reproduced bulk water properties; HBD-SCC-DFTB showed some improvement due to better hydrogen bonding energy description.
- Both SCC-DFTB methods underestimated water dimer interaction energy, leading to low heat of vaporization and elevated diffusion coefficients.
- The Zundel cation (H(5)O(2)(+)) was identified as the stable hydrated excess proton species, diffusing similarly to water.
Conclusions:
- SCC-DFTB, even with damping functions, exhibits limitations in accurately describing bulk water and hydrated proton dynamics.
- The underestimation of interaction energies impacts key thermodynamic and kinetic properties.
- Significant discrepancies exist between SCC-DFTB predictions and experimental/computational benchmarks for hydrated excess protons.
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