Grand-canonical Transition-Matrix Monte Carlo (GC-TMMC) simulations [1, 5-10] of SPC/E H2O were performed at T = 300 K in two metal-organic frameworks (MOFs). The particle number range was divided into windows advanced trial moves were performed to ensure adequate sampling at high densities. The main result of a GC-TMMC simulation is the particle number probability distribution (PNPD), which is constructed by stitching together the particle number distributions from each window. The adsorption isotherm may be determined from the PNPD . At each value of N, average total potential energies U were collected and these quantities were used to compute the isosteric heat of adsorption.
Simulations used an N-dependent set of Monte Carlo moves, as described below. Simulations were initialized with Wang-Landau Monte Carlo, the TMMC collection matrix was accumulated beginning in Wang-Landau stage 18, and the simulation switched to TMMC mode after 20 stages.
Other key simulation details common to all simulations are given below:
|Fluid Model||SPC/E H2O |
|Lennard-Jones cutoff||10A, with linear-force shift tail|
|Ewald Parameters||Set according to DL_POLY recipe , with relative tolerance 10-5|
Monte Carlo Move Set
|Total Run Time||ZIF-8: 28 days ; CuBTC: 7 days|
|Bias update freq||1.0e5|
|Physical Parameters||CODATA 2018 |
The adsorbent MOFs were reconstructed from publicly-available crystal structures and replicated to ensure that the simulation cell was at least twice the cutoff radius in all dimensions. Forcefields for each MOF were taken from published literature. The MOF structure and forcefield are provided in FEASST particle files in the data repository associated with this page (see "Data Availability" below). Coulombic interactions were handled using the Ewald summation method [2,3] (parameters listed in the metadata files). Lorentz-Berthelot mixing rules were used to set the unlike-atom Lennard-Jones parameters.
Simulation details specific to each MOF
|Number of Windows||80||64|
|Unit Cell Replication (Nx, Ny, Nz)||(2,2,2)||(1,1,1)|
|Cubic Box Dimensions (A)||34.023240||26.3430|
|Simulation MOF Mass (amu)||21846.91||9677.91|
|Snurr ||Calero IV |
|FEASST MOF Particle||data.ZIF8_Snurr_rep222||data.CuBTC_CaleroIV_rep111|
Note: both MOFs have cubic unit cells
The result of each simulation is the PNPD, which may be used to compute the adsorption isotherm by the histogram-reweighting procedure described by Siderius, Hatch, and Shen in a forthcoming publication . The pressure for a particular chemical potential was determined from GC-TMMC simulation of the bulk SPC/E water ; the PNPD and average potential energy of the bulk fluid are also provided in the associated data repository.
Various data files used to generate the reference isotherms are available in a Git Repository: https://github.com/dwsideriusNIST/NIST_SRSW_Data/tree/master/H2O_REF_ISOTHERMS
Files in the repository include:
FEASST particle files for the MOF materials [includes atomic coordinates and the forcefield parameters]
FEASST particle file for SPC/E Water
Particle number probability distributions for both the bulk and adsorbed water
Isotherm data files, including the adsorption isotherm and estimated uncertainties, formatted as AIF files