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The interactions of CO2/H2 with p-tert-butylcalix[4]arene (TBC4) were studied using potential of mean force (PMF) and free energy perturbation approaches. To the best of our knowledge, the present work is one of the first to employ the constrained mean force approach to evaluate solute selectivity by the TBC4 molecule. The computed PMFs for the interaction of CO2/H2 with a single TBC4 molecule establish that the interaction of CO2 with the open end of the cage structure is attractive while the interaction with H2 is repulsive. Free energy perturbation calculations were performed for the same two guest molecules with a pair of facing TBC4 molecules used as a representative model as found in the TBC4 molecular solid. At low temperature, both CO2/H2 have favorable interactions with the TBC4 pair, with the CO2 interaction being considerably greater. These results are in agreement with recent experimental data showing considerable CO2 uptake by TBC4 at moderate pressures. [ABSTRACT FROM AUTHOR]