Researchers at NIST have investigated how different methods of calculating partial charges affect the simulated adsorption properties of ZIF-8, a metal-organic framework known for its ability to capture carbon dioxide.
The study used the Quantum ESPRESSO electronic structure calculator to compute the electronic structure of ZIF-8. Various methods were employed to calculate partial charges from the resulting electron density maps. Monte Carlo molecular simulations were then performed to obtain the adsorption isotherms of CO2 in ZIF-8.
The research team optimized the lattice constants of ZIF-8 using damped (Beeman) dynamics of the Wentzcovitch extended Lagrangian. XYZ-formatted files were modified to be compatible with standard XYZ format, with each atom having a Lennard-Jones site and partial charge.
The adsorption of CO2 by ZIF-8 was modeled using the TraPPE force field with Lennard-Jones parameters and partial charges. Constituent atoms in ZIF-8 were modeled using a force field introduced by Perez-Pellitero et al., with cross terms computed using standard Lorentz-Berthelot mixing rules.
Adsorption simulations were run using the Transition-matrix Monte Carlo technique and analyzed using histogram-reweighting, with the equation of state of CO2 obtained from bulk simulations. The study found that partial charges may not sum to zero, and adjusted partial charges were listed for further analysis.
Keywords: quantum, computing, algorithms
