BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 137

MD studies. However, in general whenever experimental equilibrium properties
such as adsorption isotherms have been reproduced by the molecular simulations,
it has been observed that dynamic simulations based on the same interatomic
potentials are also reliable. Therefore, many MD studies examining gas diffusion
in MOFs first showed the good agreement between experiments and simulations
for gas adsorption isotherms and then used the same potential models for gas
diffusion simulations. Here, it is useful to highlight that considering a wide gas
loading range when comparing simulation results with experimental data is cru-
cial. It is unreasonable tocompare outcome of simulations with the experimental
measurements over a very narrow range of loading and assumes that good (or
poor) agreement with experiment will continue to high loadings. The MD simu-
lations have used general-purpose force fields. The MD simulations have used
general-purpose force fields such as the universal force field (UFF), DREIDING
force field( and optimized potential for liquid simulations all-atom (OPLS-AA)
force field for representing the interactions between MOF atoms and adsorbates.
There are studies where the parameters of the force fields are refined to match the
predictions of simulations with the experimental measurements (in most cases
experimental adsorption isotherm data existwhereas experimental diffusion data
do not exist) or using first principles calculations. Of course, one must be careful
in refining force field parameters to match the results of simulations with the ex-
perimental data since the accuracy of the experiments are significantly affected by
the defects of as synthesized MOFs or trapped residual solvent molecules present
in the samples. Most MD simulations performed to date have assumed rigid MOF
structures, which mean the framework atoms are fixed at their crystallographic
positions. Generally, the crystallographic data for MOFs are obtained from X-ray
diffraction experiments. In rigid framework simulations, only the non-bonding
parameters, describing the pair wise interactions between the adsorbate and the
adsorbent atoms of the particular force field, were used. It can be anticipated
that the assumption of a rigid framework brings a huge computational efficiency
yet the inclusion of the lattice motion and deformation is crucial for an accurate
description of diffusion of large gas molecules since they fit tightly in the MOF
pores, forcing the MOF to deform in order to allow migration from pore to pore.
The literature summary presented so far indicates that the number of MD simula-
tion studies with flexible MOFs and flexible force fields is very limited. More
research will sure be helpful to understand the importance of lattice dynamics on
diffusivity of gas molecules in MOFs. Studies to date indicated that the lattice dy-
namics are specifically important in computing diffusivity of large gas molecules
(such as benzene) in MOFs having relatively narrow pores. Studies on flexible
force fields also suggested that a force field developed for a specific MOF can be
adapted to similar MOF structures (as in the case of IRMOFs) with slight modifi-
cations for doing comparative studies to provide a comprehensive understanding
of gas diffusion in flexible MOFs.