With the development of nuclear power, increasing attention is being paid to the treatment of nuclear waste gas, including (CH4)-C-14 . Therefore, it is desirable to develop high-capacity (CH4)-C-14 adsorption materials. Metal-organic frameworks (MOFs) are an appropriate choice due to their high capacity and tunable properties. MOF PCN-14 is among the best (CH4)-C-14 adsorption materials. Furthermore, uptake in PCN-14 can potentially be tuned by altering the (CH4)-C-14 MOF interaction through metal substitution on open metal sites (OMS). Predicting the impact of metal substitution remains a challenge, however, because general interatomic potentials commonly used in calculating uptake do not properly describe interactions involving OMS. Here, a new interatomic potential that explicitly accounts for these interactions is derived from quantum-mechanical calculations. The potential reproduces both the measured (CH4)-C-14 isotherm for PCN-14 and the site preference for adsorption at OMS. Extension to 5 metalsubstituted variants confirms that OMS composition can dramatically alter uptake, with Ni- and Ca-based compounds predicted to exceed the performance of Cu-PCN-14 largely. Trends in (CH4)-C-14 uptake correlate well with elementary MOF properties such as surface area, binding energy between metal site and (CH4)-C-14 molecule.