A calculation method for Henry constants of diatomic molecules in zeolite cages is proposed. The method is based on a molecular electrostatic potential approximation formulated within a density functional theory (DFT) context for the evaluation of interaction energies. This approach is compared with ab initio and “molecule-in-point-charge-field” calculations. The effect of wave function versus DFT methods and basis set is discussed. The influence of the integration grid is studied and the partitioning of the cage in different peels, depending on the level of calculation, is considered. An optimal “quality/cost” ratio for the Henry and separation constants for nitrogen, oxygen, and argon (included as reference) in an NaY zeolite α-cage is obtained using the B3LYP/6-31G* method for the evaluation of the interaction energies and a Van der Waals radii-based criterion for determining the integration grid. Including carbon monoxide as a fourth example, for which no experimental results were yet available, confirms the excellent results for the calculation of separation constants. Some further refinement for the evaluation of adsorption energies is necessary. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 909–922, 2000