Adsorption isotherms are fundamental quantities of interest for understanding separation and catalysis phenomena. They are typically computed by grand-canonical or Gibbs ensemble Monte Carlo methods. These methods rely on potentially challenging particle insertion/deletion moves and may present a particular challenge for simulations using a first-principles potential energy surface. As an attractive alternative, the two-phase thermodynamic (2PT) model1
allows one to calculate the free energy of a system from vanilla canonical molecular dynamics simulations. The 2PT method decomposes the total density of states into a gas-like component and a solid-like component. The former accounts for all fluidic effects and is treated as a hard-sphere gas. The solid-like component represents the non-zero, higher frequency modes and is handled as harmonic oscillators, which allows for straightforward quantum corrections by using the appropriate statistics. The 2PT method has been demonstrated to be capable of calculating the vapor-liquid coexisting properties and has recently been applied to study the free energy and stability of molecules adsorbed in carbon nanotubes2
. In this talk, we will describe the development of the 2PT method for calculating adsorption isotherms and present the results for a united-atom methane model and the TIP4P water. We will discuss important caveats with the direct application of the original 2PT method and the modifications to the method that are necessary to obtain a correct description of adsorption equilibria.
(1) Lin, S.-T.; Blanco, M.; Goddard, W. A. The Two-Phase Model for Calculating Thermodynamic Properties of Liquids from Molecular Dynamics: Validation for the Phase Diagram of Lennard-Jones Fluids. J. Chem. Phys. 2003, 119, 11792â11805.
(2) Kumar, H.; Mukherjee, B.; Lin, S.-T.; Dasgupta, C.; Sood, A. K.; Maiti, P. K. Thermodynamics of Water Entry in Hydrophobic Channels of Carbon Nanotubes. J. Chem. Phys. 2011, 134, 124105.
(3) Bukowski, B. C.; Bates, J. S.; Gounder, R.; Greeley, J. DefectâMediated Ordering of Condensed Water Structures in Microporous Zeolites. Angew. Chem. Int. Ed. 2019, 58, 16422â16426.