Quantification of Anisotropy in Exchange and Dispersion Interactions: A Simple Model for Physics-Based Force Fields

Area of Science:

• Computational chemistry
• Molecular modeling
• Intermolecular forces

Background:

• Empirical models often assume spherical symmetry for exchange repulsion, limiting accuracy.
• Quantum chemical methods can explicitly treat orientation-dependent exchange, but are computationally expensive.
• Accurate modeling of anisotropic exchange is crucial for understanding molecular interactions.

Purpose of the Study:

• To quantify the anisotropy of exchange and dispersion energy in hydrogen halides and water.
• To develop improved empirical models for representing orientation-dependent exchange interactions.
• To reduce errors in molecular simulations using enhanced force fields.

Main Methods:

• Symmetry-Adapted Perturbation Theory (SAPT) was used to probe hydrogen halides and water with a helium atom.
• Anisotropy of exchange and dispersion energy was calculated based on probe location.
• Empirical force field models were modified using angle-dependent potentials and virtual sites.

Main Results:

• Exchange interaction was found to be orientation dependent, reduced by up to 33% in hydrogen iodide due to the σ-hole.
• Lone pairs and σ-holes on water significantly modulated the exchange interaction.
• Modeling anisotropy with virtual sites reduced empirical model errors by a factor of 5 for hydrogen halides and 80% for water.

Conclusions:

• Orientation-dependent exchange repulsion is significant in hydrogen halides and water.
• Virtual sites effectively capture anisotropic exchange interactions, improving empirical force field accuracy.
• This work provides a pathway for more precise molecular simulations through better force field parameterization.