However, there are a number of limitations introduced by GCMC simulations that the reader should bear in mind. Ranking materials for a given application in a database based on GCMC data assumes that host-guest interactions are equivalently described with the same accuracy for all host atoms. However this is rarely the case, in particular for polar gas molecules and/or open metal sites on frameworks. Most studies rely on "general" or "universal" classical forcefield, such as UFF, Dreiding and TraPPE, to model van der Waals interactions between the atoms of the frameworks and the atoms of the gas. Electrostatic interactions are then calculated from partial atomic charges derived using semi-empirical methods, ML methods, or DFT calculations on large set of structures. Ab initio FFs might be necessary to improve the quality of generic FFs: 61 in particular, ab initio FFs have been used to describe accurately the adsorption uptakes of polar molecules in frameworks with open-sites metal. They are, however, computationally very expensive.