The adsorption Of Pure O-2 and N-2 and their mixture in C-168 schwarzite, as a model for a nanoporous carbon adsorbent, has been studied with use of grand canonical Monte Carlo simulations. Th. gas-carbon interaction is modeled by a pairwise additive atom-atom Lennard-Jones potential with parameters determined from ab initio quantum mechanical computations. For pure O-2, the adsorption simulated with the ab initio potential is similar to that with the empirical Steele potential derived from gas adsorption on planar graphite in the limit of zero loading; however, for pure N-2, the adsorption simulated with the ab initio potential is much larger. Consequently, a large adsorption selectivity of N-2 over O-2 from their mixture is found with the ab initio potential. With both potentials the adsorbed gas molecules are found to preferentially align along the channel intersection of the C-168 schwarzite structure, and a localized nonuniform density distribution of the adsorbed gas molecules is observed, which is more evident with the ab initio potential. The predictions of mixture adsorption using the ideal-adsorbed-solution theory based on the adsorption of only the pure gases agree well with the simulations. This work demonstrates the importance of an accurate adsorbate-adsorbent interaction potential in the determination of gas adsorption behavior, and suggests that nanoporous carbon membranes might be useful for air separation.