Using test particle sum rules to construct accurate functionals in classical Density Functional Theory

TL;DR

This paper enhances classical density functional theory for hard-sphere fluids by introducing sum rules to better determine free parameters, leading to improved predictive accuracy and a potential testing framework for DFT approximations.

Contribution

It introduces and employs two statistical mechanical sum rules to fix free parameters in FMT, improving its accuracy for fluid phase predictions.

Findings

Sum rules help fix FMT parameters more accurately.

Improved predictions of fluid properties.

Potential for testing general DFT approximations.

Abstract

Fundamental Measure Theory (FMT) is a successful and versatile approach for describing the properties of the hard-sphere fluid and hard-sphere mixtures within the framework of classical density functional theory (DFT). Lutsko [Phys. Rev. E 102, 062137 (2020)] introduced a version of FMT containing two free parameters, to be fixed by additional physical constraints. Whereas Lutsko focused on the stability of crystalline phases, we introduce and employ two statistical mechanical sum rules pertinent for the fluid phase, that are not automatically satisfied by FMT. By minimizing the relative deviation between different routes to calculate the excess chemical potential and the isothermal compressibility we determine the two free parameters of the theory. Our results indicate that requiring consistency with these sum rules can improve the quality of predictions of FMT for properties of the hard-sphere fluid phase. We suggest that employing these (test particle) sum rules, which apply for any interparticle pair-potential, might provide a means of testing the performance and accuracy of general DFT approximations.