Bridging hydroxyl groups in faujasite: Periodic vs cluster density functional calculations

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