2.5 Theoretical calculations
GCMC simulations were carried out using the Sorption program (BIOVIA
Material studio 2019) to investigate the interactions between the
framework and gas molecules. In our work, the crystal structures of
MIL-120Al were chosen for our related simulations without further
geometry optimization. A cut-off radius of 18.5 Å was used to handle the
non-bonding interactions and the Ewald & Group summation method was
applied to calculate the long-range electrostatic interactions. Each
state point of the GCMC simulations contained 1 × 107steps to guarantee equilibration, followed by 1 × 107steps to sample the required thermodynamics properties. The MOF
frameworks were treated as rigid structures by fixing the atoms at their
crystallographic positions, which has been proven to yield accurate
results in a large variety of GCMC studies. COMPASSII force field was
adopted to describe the interactions for the atoms in our simulations.
The partial charges for all atoms were derived using the DFT method
(GGA-PBE/DNP).41