Zeolites are important in many catalytic and separation processes in industry. For an optimal use of these catalytic properties it is necessary to gain a good knowledge about the structure of zeolites. Theory can help to explain the catalytic properties of zeolites. The group of Prof. J. Sauer (Berlin) has made many contributions to the explanation of the reactive behaviour of zeolitic catalysts with quantum-chemical calculations. To obtain reliable answers from these calculations one has to include electron-correlation effects. A new and attractive way to do this is density functional theory (DFT). The use of DFT should enable us to calculate much larger systems. As an Human-Capital-and-Mobility category 30 fellow in Cambridge I have worked in the group of Prof. N. C. Handy on functionals in DFT. I believe that my experience with DFT is such that I can make a useful contribution to the research of the Sauer group.