Accurate first-order perturbation theory for fluids: uf-theory

Area of Science:

• Thermodynamics
• Statistical Mechanics
• Physical Chemistry

Background:

• Accurate thermodynamic descriptions of simple fluids are crucial for understanding material properties.
• Existing perturbation theories often struggle with quantitative accuracy across wide density ranges.
• Bridging the gap between low-density and high-density fluid behavior remains a challenge.

Purpose of the Study:

• To develop a novel first-order perturbation theory for near-quantitative thermodynamic description of simple fluids.
• To establish rigorous upper and lower bounds for Helmholtz free energy.
• To create an accurate and broadly applicable interpolation scheme for fluid thermodynamics.

Main Methods:

• Utilizing a first-order Mayer-f expansion as a lower bound for Helmholtz free energy.
• Employing a first-order u-expansion to provide a rigorous upper bound for Helmholtz free energy.
• Developing a density-dependent interpolation scheme combining both bounds with two adjustable parameters.

Main Results:

• The proposed theory provides a near-quantitative description of simple fluid thermodynamics.
• Accurate representation of full fluid-phase behavior for Lennard-Jones fluids was achieved.
• The interpolation scheme demonstrated transferability to Mie m-6 fluids and simple mixtures.

Conclusions:

• The developed perturbation theory offers a robust and accurate method for calculating fluid thermodynamics.
• The interpolation scheme effectively bridges low- and high-density regimes.
• The theory's applicability to various potentials and mixtures highlights its practical utility.