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We present a systematic and efficient methodology to derive accurate (nonpolarizable) force fields from periodic density functional theory (DFT) calculations for use in classical molecular simulations. The methodology requires reduced computation cost compared with other conventional ways. Moreover, the whole process is performed self-consistently in a fully periodic system. The force fields derived by using this methodology nicely predict the CO2 and H2O adsorption isotherms inside Mg-MOF-74, and is transferable to Zn-MOF-74; by replacing the Mg-CO2 interactions with the corresponding Zn-CO2 interactions, we obtain an accurate prediction of the corresponding isotherm. We have applied this methodology to address the effect of water on the separation of flue gases in these materials. In general, the mixture isotherms of CO2 and H2O calculated with these derived force fields show a significant reduction in CO2 uptake with the existence of trace amounts of water vapor. The effect of water, however, is found to be quantitatively different between Mg- and Zn-MOF-74.