Simulating X-ray Absorption Spectroscopy in Challenging Environments: Methodological Insights from Water-Solvated Ammonia and Ammonium Systems.

Core-electron excitations in solvated systems, influenced by solvent geometry and hydrogen bonding, make X-ray absorption spectroscopy (XAS) a valuable tool for assessing solvent-solute interactions. However, calculating XAS spectra with electronic-structure methods has proven challenging due to a delicate interplay between correlation and solvation effects. This study provides a computational procedure for XAS modeling in solvated systems, with water-solvated ammonia and ammonium systems serving as probes. Exploring methodological challenges, we investigate explicit embedding models, specifically the polarizable embedding family, including polarizable density embedding and extended polarizable density embedding. Our linear-response time-dependent density functional theory (LR-TDDFT) XAS calculations reveal the efficiency of this approach, with extended polarizable density embedding emerging as a robust improvement over polarizable density embedding. Contrary to some recent literature, our study challenges the belief that LR-TDDFT cannot accurately describe XAS spectra of ammonia and ammonium solvated in water.

Accuracy of One- and Two-Photon Intensities with the Extended Polarizable Density Embedding Model.

The recently developed extended polarizable density embedding (PDE-X) model is evaluated for the spectroscopic properties of organic chromophores solvated in water, including both one- and two-photon absorption properties. The PDE-X embedding model systematically improves vertical excitation energies over the preceding polarizable density embedding model (PDE). PDE-X shows more modest improvements over existing embedding models for oscillator strengths and two-photon absorption cross-sections, which are more sensitive properties. We argue that the origin of these discrepancies is related to the description of polarization effects, suggesting directions for future development of the embedding model.

Density Functional Tight Binding Theory Approach for the CO2 Reduction Reaction Paths on Anatase TiO2 Surfaces.

Herein, we have investigated the CO2 reduction paths on the (101) anatase TiO2 surface using an approach based on the density functional tight binding (DFTB) theory. We analyzed the reaction paths for the conversion of carbon dioxide to methane by performing a large number of calculations with intermediates placed in various orientations and locations at the surface. Our results show that the least stable intermediate is CO2H and therefore a key bottleneck is the reduction of CO2 to formic acid. Hydrogen adsorption is also weak and would also be a limiting factor, unless very high pressures of hydrogen are used. The results from our DFTB approach are in good agreement with the hybrid functional based density functional theory calculations presented in the literature.