Comparing polarizable force fields to ab initio calculations reveals nonclassical effects in condensed phases

Area of Science:

• Computational Chemistry
• Molecular Modeling
• Quantum Chemistry

Background:

• Many-body force field models often fail to accurately reproduce both gas-phase and condensed-phase molecular polarizability.
• Previous studies indicated that polarizable force fields can overpolarize systems like bifurcated water chains compared to ab initio calculations.
• This overpolarization was attributed to the omission of coupling between many-body exchange and polarization effects.

Purpose of the Study:

• To reproduce and validate previous findings on polarizable force field limitations using various ab initio methods.
• To investigate the role of intermolecular charge transfer in hydrogen-bond forming systems.
• To determine the dominant polarization effects in water under standard conditions.

Main Methods:

• Utilized different levels of ab initio theory for validation.
• Employed a polarizable force field model based on the chemical-potential equalization principle.
• Analyzed both bifurcated water chains and hydrogen-bonded systems, including water at standard conditions.

Main Results:

• Reproduced the overpolarization of bifurcated water chains observed in previous studies.
• Demonstrated that intermolecular charge transfer significantly impacts polarizable force fields in hydrogen-bonded systems.
• Showed that intermolecular charge transfer leads to underpolarization, contrasting with the effect of many-body exchange coupling.
• Identified intermolecular charge transfer as the dominant effect in water at standard conditions due to the prevalence of hydrogen-bonded geometries.

Conclusions:

• Classical polarizable force fields require inclusion of intermolecular charge transfer for accurate modeling of hydrogen-bonded systems.
• The neglect of intermolecular charge transfer leads to underpolarization in systems like water.
• Accurate molecular simulations necessitate accounting for both many-body exchange-polarization coupling and intermolecular charge transfer.