Acidifying the Madrid-2019 force field: A rigid model for H3O+ with scaled charges

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Developing accurate molecular models for ions is crucial for understanding electrolyte solutions.
• Classical force fields require careful parameterization to capture ion-specific behaviors.
• The Madrid-2019 model introduced scaled charges for ions to improve simulations.

Purpose of the Study:

• To introduce a classical, rigid force field for the oxonium cation (H3O+) optimized in TIP4P/2005 water.
• To investigate the effects of scaled charges on the behavior of H3O+ in various aqueous solutions.
• To provide a computationally efficient model for studying acidic electrolyte solutions.

Main Methods:

• A classical and rigid force field for H3O+ was optimized.
• Charges for H3O+ and counterions (Cl-, Br-, I-, NO3-) were scaled by 0.85, following the Madrid-2019 model.
• The charge distribution of H3O+ was derived using the self-consistent atomic dipole-corrected Hirshfeld approach.
• Molecular dynamics simulations were performed to compare with experimental data.

Main Results:

• The model shows remarkable agreement with experimental data for solution density as a function of solute concentration.
• Scaled charges accurately describe the local structure of hydrogen halides.
• Limitations were observed in capturing ion-pairing and long-range hydrogen bonding dynamics.
• The scaled charge of 0.85e was detrimental to predicting solution viscosity for some electrolytes but not for HNO3.

Conclusions:

• The optimized H3O+ force field expands the Madrid-2019 family of ions with scaled charges.
• The model offers a computationally efficient platform for studying acidic electrolyte solutions.
• Findings advance molecular modeling techniques for understanding the interplay between local structure and transport properties.