High-throughput computational screening is an increasingly useful approach to identify promising nanoporous materials for gas separation and adsorption applications. The reliability of the screening hinges on the accuracy of the underlying force fields, which is often difficult to access systematically. To probe the accuracy of common force fields and to assess the sensitivity of the screening results to this accuracy, we have computed CO2 and CH4 gas adsorption isotherms in 424 metal-organic frameworks using ab initio force fields and evaluated the contribution of electrostatic, van der Waals, and polarization interactions on the predicted gas uptake and the adsorption site probability distributions. While there are significant quantitative differences between gas uptake predicted by standard (generic) force fields (such as UFF) and ab initio force fields, the force fields predict similar ranking of the MOFs, supporting the further use of generic force fields in high-throughput screening studies. However, we also find that isotherm predictions of standard force fields may benefit from significant error cancellation resulting from overestimation of dispersion and neglect of polarization; as such, caution is warranted, as this error cancellation may vary among different classes of materials.