We have developed force fields for the calculation of adsorption of NH3, CO2, and H2O on zeolite 4A by performing Gibbs ensemble Monte Carlo simulations to fit experimental isotherms at 298 K. The calculated NH3 and CO2 isotherms are in excellent agreement with experimental data over a wide range of temperatures and several orders of magnitude in pressure. We have calculated isotherms for H2O in 4A using two different models and have found that H2O saturates zeolite 4A even at pressures as low as 0.01 kPa for the range of temperatures studied. We have studied the geometry of the adsorption sites and their dependence on loading. At low pressures, CO2 molecules adsorb with their longitudinal axis pointing toward the center of the supercage, whereas at higher pressures, the two oxygen atoms are equidistant from the Na atom in the binding site.