Polarizable shell-model potentials are widely used for atomic-scale modeling of charged defects in solids using the Mott–Littleton approach and hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) embedded-cluster techniques. However, at the pure MM level of theory, the calculated defect energetics may not satisfy the requirement of quantitative predictions and are limited to only certain charged states. Here, we proposed a novel interatomic potential development scheme that unifies the predictions of all relevant charged defects in CeO2 based on the Mott–Littleton approach and QM/MM electronic-structure calculations. The predicted formation energies of oxygen vacancies accompanied by different excess electron localization patterns at the MM level of theory reach the accuracy of density functional theory (DFT) calculations using hybrid functionals. The new potential also accurately reproduces a wide range of physical properties of CeO2, showing excellent agreement with experimental and other computational studies. These findings provide opportunities for accurate large-scale modeling of the partial reduction and nonstoichiometry in CeO2, as well as a prototype for developing robust interatomic potentials for other defective crystals.

The copper-exchanged zeolite Cu-CHA has received considerable attention in recent years, owing to its application in the selective catalytic reduction (SCR) of NO x species. Here, we study the NH3-SCR reaction mechanism on Cu-CHA using the hybrid quantum mechanical/molecular mechanical (QM/MM) technique and investigate the effects of solvent on the reactivity of active Cu species. To this end, a comparison is made between water- and ammonia-solvated and bare Cu species. The results show the promoting effect of solvent on the oxidation component of the NH3-SCR cycle since the formation of important nitrate species is found to be energetically more favorable on the solvated Cu sites than in the absence of solvent molecules. Conversely, both solvent molecules are predicted to inhibit the reduction component of the NH3-SCR cycle. Diffuse reflectance infrared fourier-transform spectroscopy (DRIFTS) experiments exploiting (concentration) modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) identified spectroscopic signatures of Cu-nitrate and Cu-nitrosamine (H2NNO), important species which had not been previously observed experimentally. This is further supported by the QM/MM-calculated harmonic vibrational analysis. Additional insights are provided into the reactivity of solvated active sites and the formation of key intermediates including their formation energies and vibrational spectroscopic signatures, allowing the development of a detailed understanding of the reaction mechanism. We demonstrate the role of solvated active sites and their influence on the energetics of important species that must be explicitly considered for an accurate understanding of NH3-SCR kinetics.

Hybrid quantum mechanical/molecular mechanical (QM/MM) methods are a powerful computational tool for the investigation of all forms of catalysis, as they allow for an accurate description of reactions occuring at...

BiVO 4 (BVO) is an important photocatalytic and ferroelastic material. It has been extensively studied using density functional theory (DFT). However, on optimization, at a commonly employed level of theory using the Perdew−Burke−Ernzerhof (PBE) exchange− correlation functional, the monoclinic scheelite (ms-BVO) structure transforms into a highersymmetry tetragonal scheelite (ts-BVO) phase spontaneously, which has also been confirmed by other groups. Such a transformation is highly unusual, as one would expect the transition to a lower symmetry structure to be modeled well at this level of theory, as is the case with, for example, the perovskite BaTiO 3 , and hints at a subtle interplay between structural and electronic properties. In this work, we demonstrate that this phase transition nevertheless can be described accurately with DFT but only using a hybrid density functional with ∼60% Hartree−Fock (HF) exchange. We find a soft phonon mode in ts-BVO, which corresponds to the phase transition from ts-BVO to ms-BVO associated with a double-well potential characterizing this phase transition, implying that the transition is of the second order. We find two key factors that can explain this surprising behavior. First, the polarizability of the Bi 3+ ion, with an on-site contribution from the hybridization of its 6s and 6p states, is notably underestimated by DFT. Moreover, the effective radius of the Bi 3+ ion proves to be too large. With the 60% HF exchange hybrid functional, the description of the polarizability of Bi 3+ does not improve but the radii of the Bi 3+ ions approach more realistic values. The polarizability of the O and V ions are reasonably described already by PBE. To gain further insight into the problem, we investigated the structural stability of other ABO 4 oxides, including ScVO 4 , LaNbO 4 , YTaO 4 , and CaWO 4 , and related materials. Some of them have similar behavior to BVO, whose ground-state monoclinic structure proves to be unstable using commonly employed DFT approaches. In particular, for ScVO 4 , we found that the scheelite tetragonal and fergusonite monoclinic structures cannot be distinguished using the PBEsol functional. But the fergusonite monoclinic structure becomes stable using the hybrid functionals with high fractions of HF exchange, which points to the crucial role of the accurate ionic size reproduction by the method of choice as the on-site Sc 3+ polarizability is too low to have a significant effect. Our findings would be of high interest for the study of other problematic materials with subtle size and polarization properties, especially ABO 4 oxides that undergo similar phase transitions.

Cu impurities are reported to have significant effects on the electrical and optical properties of bulk ZnO. In this work, we study the defect properties of Cu in ZnO using hybrid quantum mechanical/molecular mechanical (QM/MM)–embedded cluster calculations based on a multi-region approach that allows us to model defects at the true dilute limit, with polarization effects described in an accurate and consistent manner. We compute the electronic structure, energetics, and geometries of Cu impurities, including substitutional and interstitial configurations, and analyze their effects on the electronic structure. Under ambient conditions, CuZn is the dominant defect in the d9 state and remains electronically passive. We find that, however, as we approach typical vacuum conditions, the interstitial Cu defect becomes significant and can act as an electron trap.