See Methods for details about both molecular simulation techniques. Given access to (only) these two molecular simulation techniques that trade fidelity and computational runtime, we reframe the objective as: find the COF x ∈ X with the highest adsorptive Xe/Kr selectivity according to the highfidelity BGCMC simulation, y (2/3) , while incurring the minimal computational cost, measured by the sum of run times of the (both low-and highfidelity) simulations we conduct to find x .