Studying vapor-liquid transition using a generalized ensemble

Area of Science:

• Thermodynamics
• Statistical Mechanics
• Computational Chemistry

Background:

• Homogeneous vapor-liquid nucleation is crucial for phase transitions but challenging to simulate due to unstable intermediate states.
• Traditional methods like canonical ensemble simulations struggle to access and sample these critical nucleation pathways.
• Accurate calculation of nucleation barriers and rates is essential for understanding diverse physical and chemical processes.

Purpose of the Study:

• To investigate homogeneous vapor-liquid nucleation using a generalized Replica Exchange Method (gREM).
• To efficiently sample unstable nucleation states and obtain a continuous free energy profile from vapor to liquid.
• To calculate the nucleation free energy barrier and rate without relying on phase classification schemes.

Main Methods:

• Utilized the generalized Replica Exchange Method (gREM) for enhanced sampling of molecular systems.
• Employed the Statistical Temperature Weighted Histogram Analysis Method (SWHAM) for free energy curve reconstruction.
• Simulated planar, cylindrical, and spherical interfaces within a single gREM simulation.
• Calculated excess Gibbs free energy for spherical droplet formation in a Lennard-Jones system.

Main Results:

• gREM enabled efficient sampling of multiple states, including unstable nucleation pathways.
• A continuous free energy curve was obtained, allowing direct calculation of the nucleation barrier.
• The calculated nucleation free energy barrier for a spherical droplet agreed with umbrella sampling results.
• The nucleation rate was determined directly from the free energy barrier, bypassing phase classification.

Conclusions:

• gREM provides an efficient and robust method for studying homogeneous vapor-liquid nucleation.
• The approach allows for accurate calculation of nucleation barriers and rates without arbitrary phase definitions.
• This methodology offers a significant advancement for simulating phase transition phenomena.