Benchmarking DFT Calculations for Adsorption Studies: Pharmaceuticals and Related Organic Molecules in Cation-Exchanged Zeolites

The choice of the exchange-correlation functional and
dispersion
correction significantly influences the adsorption energies obtained
from dispersion-corrected density functional theory (DFT) methods.
Several studies have employed different approaches to calculate the
adsorption energy of small (and a few large) molecules in all-silica
or protonated zeolites, often referencing high-level computational
results or experimental data. However, systematic investigations into
the adsorption of large pharmaceutical and personal care product (PPCP)
contaminants in cation-exchanged zeolites remain limited. This study
aims to determine which functional is particularly well-suited for
large-scale screening of large PPCP adsorption in cation-exchanged
zeolites. We have studied nine (mono- and divalent) metal cations
(including alkali, alkaline earth, and transition metals) and compared
11 dispersion-corrected DFT approaches, including methods based on
pairwise dispersion corrections, nonlocal van der Waals, and Minnesota
density functionals. Furthermore, saturated cluster (SC) models are
used to calculate DFT energies, which are then compared with the “gold
standard” reference values from the DLPNO–CCSD(T) method.
For periodic systems, the variation in adsorption energies for host–guest
combinations is explained using cationic radii versus adsorption energy
trends, with a few exceptions further clarified through charge density
mapping, which identifies cases in which cation−π interactions
are present. On the other hand, in SC models, various error statistics,
along with the decomposition of total adsorption energies into electronic
and dispersion contributions (for DFT) and Hartree–Fock and
correlation energies (CCSD(T)), are applied to identify the most accurate
functional for PPCPs@cation-FAU complexes. Overall calculations and
error analysis provide a clear conclusion that the PBEsol-D3 functional
can be recommended for the prediction of PPCP adsorption energies
in cation-exchanged and all-silica zeolites.