Alkane adsorption in Na-exchanged chabazite: the influence of dispersion forces.

The importance of dispersion forces for the correct description of the adsorption of short alkanes in Na-exchanged and purely siliceous chabazite has been investigated at different levels of theory: (i) standard density-functional (DFT) calculations using the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional in the generalized gradient approximation, (ii) dispersion corrections based on empirical force fields according to Grimme [J. Computat. Chem. 134, 1463 (2004)- PBE-d], (iii) calculations based on the van der Waals density functional (vdW-DF) proposed by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], and (iv) using the random phase approximation (RPA) in combination with the adiabatic-connection fluctuation-dissipation theorem (RPA-ACFDT), using wave-functions calculated at the DFT and Hartree-Fock (HF) levels. A full relaxation of the adsorbate-zeolite complex was performed at the PBE, PBE-d, and vdW-DF levels. RPA and RPA-HF energies were calculated for the optimized configurations. A critical analysis of the results shows that the most accurate description is achieved at the RPA level with HF exchange energies, while both PBE-d and vdW-DF overestimate the strength of the interaction with the acid site.