Using a hierarchical multiscale approach combining quantum mechanics and molecular simulation, we have investigated the adsorption of pure CO2 and N-2 and their mixture at room temperature in C-168 schwarzite, as a model for nanoporous carbons. First, the adsorbate-adsorbent interaction potential is determined using ab initio quantum mechanics computations, and then the adsorption is predicted using full atomistic Monte Carlo simulations. The extents of adsorption, adsorption energies, and isosteric heats of pure CO2 and N-2 simulated with the ab initio potential are found to be higher than those with the empirical Steele potential that had been developed from gas adsorption on planar graphite. The inclusion of the electric quadrupole moment of adsorbate in simulation has no discernible effect on N-2 adsorption but results in a larger extent of CO2 adsorption at high coverages. The selectivity of CO2 over N-2 in the C-168 schwarzite from a model flue gas is predicted to be significantly larger with the ab initio potential than with the Steele potential. This illustrates the importance of an accurate adsorbate-adsorbent interaction potential in determining gas adsorption and suggests that nanoporous carbons might be useful for the separation of flue gases. As a comparison, the adsorption and selectivity of CO2 and N-2 in ZSM-5 zeolites are also simulated with the experimentally validated potential parameters. The selectivity in the C-168 schwarzite predicted with the ab initio or Steele potential is found to be larger than the selectivity in all-silica ZSM-5, but less than that in Na-exchanged ZSM-5 zeolites.