Benchmarking of Molecular Dynamics Force Fields for Solid-Liquid and Solid-Solid Phase Transitions in Alkanes | JoVE Visualize

Area of Science:

Thermodynamics and physical chemistry
Computational materials science
Molecular modeling and simulation

Background:

Accurate prediction of alkane solid phase transitions is vital for industrial applications like pipeline safety, lubrication, and energy storage.
Simulating these transitions is computationally challenging and often excluded from molecular force field development.
Understanding alkane phase behavior is fundamental to processes such as artificial morphogenesis.

Purpose of the Study:

To benchmark seven molecular dynamics models for their accuracy in predicting alkane phase transitions.
To compare simulation results with experimental data for liquid properties, liquid-solid, and solid-solid transitions of n-pentadecane (C15) and n-hexadecane (C16).
To develop and utilize novel order parameters for identifying crystal-rotator phase transitions in alkanes.

Main Methods:

Molecular dynamics simulations were performed using seven distinct force fields: TraPPE, PYS, CHARMM36, L-OPLS, COMPASS, Williams, and Williams 7B.
Model performance was evaluated by comparing simulated liquid properties and phase transition temperatures (melting and crystal-rotator) against experimental data.
Specialized order parameters were designed to detect and characterize the rotator phase in alkane crystalline structures.

Main Results:

All tested models overestimated alkane melting points, with deviations up to 34 K.
The PYS and Williams 7B models demonstrated the highest accuracy for melting points, with errors of only 2 K and 3 K, respectively.
United-atom models failed to accurately represent the rotator phase, while all-atom Lennard-Jones models did not exhibit it. Williams and COMPASS models, however, did reproduce the crystal-to-rotator transition, with Williams 7B being most accurate for C15.

Conclusions:

The Williams 7B and PYS force fields offer the most accurate predictions for n-alkane melting points among the evaluated models.
The choice of potential functional form (e.g., Buckingham vs. Lennard-Jones) significantly impacts the ability to capture alkane crystal-rotator phase transitions.
Optimized all-atom models, particularly Williams 7B, are essential for accurately simulating the complex solid-state phase behavior of alkanes.