On a qualitative level, it is commonly stated that a "good fit" of PPCPs into the zeolite pores is required to achieve high affinities (translating into high removal efficiencies), because a close match between the size and shape of guest molecule and pores maximizes attractive van der Waals interactions. The above analysis of the deformation energies shows that CBZ apparently does not fit well into the pores of some zeolites, especially BEA and MOR, whose pores are too narrow. It does not, however, allow for an explanation why the interaction is strongest in IFR and much weaker in ISV, despite the similar pore sizes and the negligible deformation energy in the latter system. Therefore, the question arises what defines a good fit, and how it can be evaluated. Prior to addressing this issue, two other points are worth mentioning in brief: First, hydrogen bonds from one of the H atoms of the CBZ -NH2 group to a framework oxygen atom are present in many of the lowest-energy configurations. As shown in Table , there is no correlation whatsoever between the hydrogen bond distance and the adsorption energy (the shortest hydrogen bonds on the order of 2.1 Å are found in BEA and FAU), indicating that the hydrogen bond makes only a relatively minor contribution to the total interaction. Second, an analysis of the Mulliken charges points to a certain degree of charge transfer from the framework to the CBZ molecule, with the sum of the Mulliken charges over all CBZ atoms ranging from -0.074 e for ISV to -0.113 e for AFI (Table ). Again, however, there is no apparent correlation between the extent of the charge transfer and the adsorption energy.