A density functional theory (DFT)-based method for the calculation of interaction energies in adsorption processes is proposed. The expression is based on the expansion of the energy of the adsorbed molecule with respect to the external potential perturbation introduced by adsorption in the zeolite cage. DFT interaction energies at the B3LYP/G-31G* level for the N-2... Na+, Na+... CO and CO ... Na+ systems in the gas phase were compared with first- and second-order results, resulting from the DFT expansion of the interaction energy. The distance dependence of the second-order contribution was investigated. The present method was shown to be an interesting computational strategy for calculating interaction energies when evaluating Henry constants. (C) 1998 Elsevier Science B.V. All rights reserved.