Periodic density functional calculations (DFT) on bridging hydroxyl groups
in the zeolite faujasite are performed. It is shown that force field calcul
ations as presently parametrized are not able to reproduce the correct ener
getical ordering for these groups. Embedding not only gives the right order
ing but also agrees well with the periodic calculations for geometries. OH
stretching frequencies can be obtained in very good agreement with experime
nt by periodic DFT calculations in particular if anharmonic corrections are
included. The same functionals used for the periodic calculations have bee
n employed in calculations on model clusters, and it is shown that clusters
provide a qualitative as opposed to a quantitative description of these sy
stems.