Our multi-fidelity surrogate model treats the fidelity-∈ { 1 3 , 2 3 } simulated Xe/Kr selectivity of a COF represented by x, y ( ) ∈ R, as a realization of a random variable Y ( ) (x). The surrogate model specifies a probability density for Y ( ) (x). Under this Bayesian perspective, the posterior probability density of Y ( ) (x) | D [n] at iteration n of the MFBO search reflects our beliefs, grounded by the simulation data collected thus far in the search,