Ab Initio Parametrized Force Field for the Flexible Metal-Organic Framework MIL-53(Al)

Area of Science:

• Computational Chemistry
• Materials Science
• Solid-State Chemistry

Background:

• Flexible metal-organic frameworks (MOFs) like MIL-53(Al) exhibit unique structural transitions.
• Understanding these transitions is crucial for MOF applications.

Purpose of the Study:

• To develop and validate a reliable force field for simulating the flexible MIL-53(Al) MOF.
• To investigate the intrinsic bistable behavior of MIL-53(Al).

Main Methods:

• Density functional theory (DFT) calculations on nonperiodic clusters.
• Modified Hirshfeld-I method for atomic charges.
• MM3 model for van der Waals interactions.
• New methodology for valence force field parameterization using DFT gradients and Hessian matrix elements.

Main Results:

• The proposed force field accurately predicts experimental geometries and cell parameters for both large pore (lp) and narrow pore (np) phases.
• Energy profile analysis confirms the intrinsic bistability of MIL-53(Al), showing two minima.
• The transition between lp and np phases can occur intrinsically without guest molecules.

Conclusions:

• The developed force field provides a robust tool for simulating MIL-53(Al) flexibility.
• The study confirms the inherent large pore/narrow pore phase transition in MIL-53(Al).
• Van der Waals parameters significantly influence phase stability, with a tendency for overstabilization of the narrow pore phase using MM3 dispersion.