2. SIMULATIONS METHODS
In this work, the MXene models were from the Computational 2D Materials
Database (C2DB),41 which have been calculated and
optimized except Ti2C model from the Material Project
database.42 The simulation system box used for
modeling various MXenes had approximate dimensions of 3.2×3.1×30 nm in
three directions. The MXene nanopores with different diameters were
constructed (Figure 1a ) by gradually removing the atoms closest
to the MXene center (Figure 1b ), and their diameters were
calculated using the Proeblazer 4.0 software (Figure 1c, the
pore limiting diameter (PLD) value), with He as the probe atom. In the
xy plane, periodic bounary conditions were applied, and reflective walls
were placed along z direction at both ends of the box, as illustrated inFigure 1d . The MXene was placed at z = 15 nm to devide the box
into two parts, the feed side and the permeate side. Non-equilibrium MD
(NEMD) simulations were performed to simulate the relaxation process of
gas molecules permeating from feed side to permeate side. For single gas
simulations, 100 gas molecules were initially placed on the feed side
randomly, and for mixed gases, 200 gas molecules (CH4 :
He = 1:1) were placed on the feed side randomly. In the simulation, the
atoms of MXene were fixed, while the gas molecules were allowed to
diffuse from the feed side to the permeate side freely. To calculate the
flow rate, each simulation was repeated for 5 times (with different
initial positions for gas molecules), and the averaged result was
reported. Furthermore, equilibrium MD (EMD) simulations were also
performed to calculate the potential of mean force (PMF) when gas
molecules permeating through the MXene nanopore, where both feed and
permeate sides contain 50 gas molecules initially.
The LJ parameters of MXene atoms were taken from the UFF force
field,43 with charges calculated by the Qeq method
(Table S1) . The force field (FF) parameters of
CH4 and He were from the transferable potentials for
phase equilibria (TraPPE) force field and the work of Skoulidas et
al .44, respectively. The LJ non-bonding parameters
for different type of atoms were calculated by the Lorentz-Berthelot
combination rule. These FF parameters have been proven to yield accurate
results for a wide range of simulation studies.45-47In a typical simulation, energy minimization was performed using the
conjugate gradient method, then followed by a 50 ns (if not otherwise
specified) NVT MD simulation (T = 298.15 K, Velocity-rescale
thermostat48). The cut-off distance of Lennard-Jones
(LJ) and short-range coulomb interaction were set as 1.2 nm, with
long-range coulomb interaction calculated using the reaction-field
method.45 All simulations were performed with GROMACS
version 5.1.549 and visualized with VMD
1.9.3.50 The simulation data and trajectory were
analyzed with the MDanalysis package.51