Isomolar semigrand ensemble molecular dynamics: development and application to liquid-liquid equilibria.

Area of Science:

• Computational chemistry
• Chemical engineering
• Materials science

Background:

• Calculating liquid-liquid equilibria (LLE) is crucial for understanding phase behavior in mixtures.
• Traditional methods like Monte Carlo (MC) simulations can struggle with complex molecules.
• The isomolar semigrand ensemble offers a framework for LLE calculations.

Purpose of the Study:

• To develop and apply an extended system molecular dynamics (MD) method for calculating LLE.
• To enable more efficient and accurate LLE predictions for complex molecular systems.
• To provide an alternative to MC simulations for LLE studies.

Main Methods:

• Developed an extended system molecular dynamics method for the isomolar semigrand ensemble (N-P-T-xi).
• Utilized an extended system variable to dynamically control the fugacity fraction (xi) by particle identity transformation.
• Employed two approaches for computing coexistence points: multiple-histogram reweighting and small system thermodynamics with the equal-distance/equal-area criterion.

Main Results:

• Successfully applied the method to calculate LLE for two Lennard-Jones mixtures.
• Achieved results comparable to previous Monte Carlo (MC) simulations.
• Demonstrated the method's effectiveness near the upper critical solution temperature and in its absence.

Conclusions:

• The developed extended system MD method is a viable and efficient approach for LLE calculations.
• This MD-based method is particularly advantageous for complex molecules with challenging intramolecular degrees of freedom.
• The study provides a valuable tool for phase equilibria research in chemical engineering and materials science.