A site-renormalized molecular fluid theory.

Area of Science:

• Statistical Mechanics
• Computational Chemistry
• Physical Chemistry

Background:

• Molecular fluid theories often rely on approximations for complex interactions.
• Site-site potentials are crucial for describing molecular fluids but pose theoretical challenges.
• Existing models like the reference interaction site model (RISM) have limitations.

Purpose of the Study:

• To develop a new, exact, and proper closure for the diagrammatically proper interaction site model theory.
• To introduce a framework that intrinsically includes the molecular pair distribution function.
• To improve the quantitative accuracy of theoretical predictions for molecular fluids.

Main Methods:

• Expanding orientation-dependent pair distribution functions using a topological analog of diagrammatic theory.
• Diagrammatically renormalizing molecular fluid theory.
• Developing a new closure for the diagrammatically proper interaction site model theory incorporating bridge functions.
• Utilizing an angular expansion of molecular interactions.

Main Results:

• The proposed theory is shown to be a linearized, minimal angular basis set approximation to site-renormalized molecular theory.
• The theory is equivalent to the molecular Ornstein-Zernike treatment in a specific basis set.
• Numerical results quantitatively improve upon existing models (RISM and its variants) for both homonuclear and heteronuclear models.
• Thermodynamic quantities obey exact symmetries and sum rules, even in low-order approximations.

Conclusions:

• The new theory provides a more accurate and robust framework for studying molecular fluids.
• It resolves the auxiliary site problem inherent in previous site-site theories.
• The inclusion of improper diagrams in the angular expansion offers a significant formal advancement.