Optimization of classical nonpolarizable force fields for OH(-) and H3O(+)

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Molecular Modeling

Background:

• Accurate molecular force fields are essential for simulating chemical systems.
• Existing models often struggle to reproduce experimental data for aqueous ions like H3O+ and OH-.
• The role of intramolecular electrostatics in ion solvation is a key area of investigation.

Purpose of the Study:

• To develop optimized force fields for H3O+ and OH- ions.
• To reproduce experimental solvation free energies and solution activities.
• To investigate the possibility of creating thermodynamically consistent force fields without atomic polarizability.

Main Methods:

• Optimization of partial charges on hydrogen atoms and Lennard-Jones parameters for oxygen atoms.
• Validation against experimental solvation free energies and ion activity data.
• Testing standard and modified combination rules for ion-ion interactions.

Main Results:

• Achieved accurate reproduction of experimental data for H3O+ Cl- and Na+ OH- solutions up to 1.5 mol/L.
• Determined a significantly higher partial charge (0.8 ± 0.1|e|) for H3O+ hydrogen atoms compared to standard models.
• Found an optimal partial charge of zero for OH- hydrogen atoms and identified the need for increased anion-cation Lennard-Jones radii in Na+ OH- solutions.

Conclusions:

• The optimized force fields provide a thermodynamically consistent description of aqueous H3O+ and OH- solutions.
• Intramolecular electrostatics play a critical role in accurately modeling these ions.
• Atomic polarizability is not essential for developing accurate, nonpolarizable force fields for these systems.