Modeling the interactions between polyoxometalates and their environment.

Area of Science:

• Computational Chemistry
• Materials Science
• Physical Chemistry

Background:

• Developing accurate force fields for polyoxometalates (POMs) and organic cations is crucial for simulating their behavior.
• The non-transferability of existing force fields presents a significant challenge in accurately modeling diverse chemical systems.

Purpose of the Study:

• To create a hybrid force field combining MMFF94x for organic cations and a specialized POMFF-II for polyoxometalates.
• To address force field non-transferability by introducing and optimizing a charge-scaling factor (SF).
• To validate the accuracy of the developed mixed force field against density-functional theory (DFT) calculations.

Main Methods:

• Integration of the Merck molecular force field 94x (MMFF94x) with a force field optimized for type-II POMs (POMFF-II).
• Introduction and optimization of a charge-scaling factor (SF) to adapt POMFF-II parameters to MMFF94x.
• Molecular mechanics (MM) simulations and geometry optimizations on hepta-molybdate clusters.
• Validation through comparison with density-functional theory (DFT) calculations, including analysis of various functionals for hydrogen bonding.

Main Results:

• The developed mixed MMFF94x/POMFF-II force field demonstrates promising structural accuracy for small POM clusters.
• MM geometry optimizations show reasonable agreement with DFT calculations, suggesting MM can be a viable alternative for large systems.
• The introduced charge-scaling factor (SF) improves the force field's performance, although further refinement of nonbonded terms and atomic charge models may be beneficial.

Conclusions:

• The hybrid MMFF94x/POMFF-II force field offers a computationally efficient approach for simulating POMs with organic counterions.
• MM simulations can serve as a cost-effective complement to DFT for large POM systems where quantum methods are computationally prohibitive.
• Further optimization of specific force field components is recommended for enhanced accuracy in future studies.