Spatially Resolved Thermodynamic Integration: An Efficient Method to Compute Chemical Potentials of Dense Fluids

TL;DR

SPARTIAN is a novel spatially resolved thermodynamic integration method that efficiently computes chemical potentials of dense fluids by combining atomistic and ideal gas representations, overcoming limitations of traditional particle insertion techniques.

Contribution

The paper introduces SPARTIAN, a new approach that uses a hybrid simulation with atomistic and ideal gas regions to accurately and efficiently determine chemical potentials in dense fluids.

Findings

Successfully computed chemical potentials for Lennard-Jones liquids and mixtures.

Accurately calculated excess chemical potentials for water and salt solutions.

Validated results against literature data, demonstrating method reliability.

Abstract

Many popular methods for the calculation of chemical potentials rely on the insertion of test particles into the target system. In the case of liquids and liquid mixtures, this procedure increases in difficulty upon increasing density or concentration, and the use of sophisticated enhanced sampling techniques becomes inevitable. In this work we propose an alternative strategy, spatially resolved thermodynamic integration, or SPARTIAN for short. Here, molecules are described with atomistic resolution in a simulation subregion, and as ideal gas particles in a larger reservoir. All molecules are free to diffuse between subdomains adapting their resolution on the fly. To enforce a uniform density profile across the simulation box, a single-molecule external potential is computed, applied, and identified with the difference in chemical potential between the two resolutions. Since the reservoir is represented as an ideal gas bath, this difference exactly amounts to the excess chemical potential of the target system. The present approach surpasses the high density/concentration limitation of particle insertion methods because the ideal gas molecules entering the target system region spontaneously adapt to the local environment. The ideal gas representation contributes negligibly to the computational cost of the simulation, thus allowing one to make use of large reservoirs at minimal expenses. The method has been validated by computing excess chemical potentials for pure Lennard-Jones liquids and mixtures, SPC and SPC/E liquid water, and aqueous solutions of sodium chloride. The reported results well reproduce literature data for these systems.