Structure and bonding of water molecules in zeolite hosts: Benchmarking plane-wave DFT against crystal structure data

Abstract

Density-functional theory (DFT) calculations are widely employed to study the interaction of water molecules with zeolite frameworks. However, there have been only few attempts to assess whether these computations reproduce experimental structure data sufficiently well, especially with regard to the hydrogen positions of the water molecules. In this work, a detailed comparison between experimental crystal structures and DFT-optimised structures is made for six water-loaded natural zeolites. For each system, high-quality structure determinations from neutron diffraction data have been reported (bikitaite/Li–BIK, edingtonite/Ba–EDI, gismondine/Ca–GIS, scolecite/Ca–NAT, natrolite/Na–NAT, yugawaralite/Ca–YUG). Using a plane-wave DFT approach, the performance of six pure and three dispersion-corrected exchange-correlation functionals is compared, focusing on an optimisation of the atomic coordinates in a fixed unit cell (with cell parameters taken from experiment). It is found that the PBE and the PW91 functional give the smallest overall deviation between experiment and computation. Of the dispersion-corrected approaches, the PBE–TS functional exhibits the best performance. For the PBE and PBE–TS functionals, the agreement between experiment and DFT is analysed in more detail for different groups of interatomic distances. Regarding the OW–H distances in the water molecules, the DFT optimisations lead to physically realistic bond lengths. On the other hand, DFT has a systematic tendency to underestimate the length of hydrogen bonds. The cation-oxygen distances are mostly in very good agreement with experiment, although some exceptions indicate the necessity of further studies.