(180c) Improved Implementation of Kirkwood-Buff Solution Theory in Periodic Molecular Simulations

Kirkwood-Buff (KB) solution theory is a means to obtain certain thermodynamic derivatives from knowledge of molecular distributions. These thermodynamic properties include isothermal compressibility, partial molar volumes, and chemical potential derivatives. Traditional KB theory requires that one make integrals over radial distribution functions; these integrals suffer inaccuracies due to finite-distance truncation effects and their use in closed systems. In this work we discuss how best to minimize these inaccuracies under traditional KB theory. In addition we implement a new method for calculating KB quantities in molecular simulations with periodic boundary conditions and particularly within the canonical ensemble. The new method is based on a finite-Fourier-series expansion of molecular concentration fluctuations and leads to more reliable results for a given computational effort. The procedure is validated and compared to the original method for a nonideal liquid mixture of Lennard-Jones particles intended to imitate a real system, carbon tetrafluoride and methane.