The description of the fluid for the external potential depends on the inclusion of the Coulombic interactions: if these interactions are included, the parameters used for describing the adsorbate molecule need to resolve the orientations, which is not the case for PC-SAFT parameters. Therefore, we use the TraPPE force field to describe CO 2 in the calculation of the external potential. For the other adsorbates, we consider two options: the set of nonpolar PC-SAFT parameters by Gross and Sadowski and the set of polar PC-SAFT parameters by Gross . The non-polar set of parameters neglects the multipolar term of Eq. 2, whereas it is included for the polar set of parameters. As CH 4 is a non-polar molecule, we use the set of non-polar parameters. For N 2 , we want to be consistent in choosing PC-SAFT parameters for calculating the external potential and for the Helmholtz energy functional. In the Supplementary Materials, Sec. S2B, we compare these two sets of parameters and show that the two options produce similar predictions. Therefore, for simplicity, we choose the set of non-polar parameters.