Evaluating the Impact of the Tamm–Dancoff Approximation on X-ray Spectrum Calculations

The impact of the Tamm–Dancoff approximation (TDA) for time-dependent density functional theory (TDDFT) calculations of X-ray absorption and X-ray emission spectra (XAS and XES) is investigated, showing small discrepancies in the excitation energies and intensities. Through explicit diagonalization of the TDDFT Hessian, XES was considered by using full TDDFT with a core-hole reference state. This has previously not been possible with most TDDFT implementations as a result of the presence of negative eigenvalues. Furthermore, a core–valence separation (CVS) scheme for XES is presented, in which only elements including the core-hole are considered, resulting in a small Hessian with the dimension of the number of remaining occupied orbitals of the same spin as the core-hole (CH). The resulting spectra are in surprisingly good agreement with the full-space counterpart, illustrating the weak coupling between the valence–valence and valence–CH transitions. Complications resulting from contributions from the discretized continuum are discussed, which can occur for TDDFT calculations of XAS and XES and for TDA calculations of XAS. In conclusion, we recommend that TDA be used when calculating X-ray emission spectra, and either CVS-TDA or CVS-TDDFT can be used for X-ray absorption spectra

A nuclear configuration interaction approach to study nuclear spin effects : an application to ortho- and para-3He2@C60

We introduce a non-orthogonal configuration interaction approach to investigate nuclear quantum effects on energies and densities of confined fermionic nuclei. The Hamiltonian employed draws parallels between confined systems and many-electron atoms, where effective non-Coulombic potentials represent the interactions of the trapped particles. One advantage of this method is its generality, as it offers the potential to study the nuclear quantum effects of various confined species affected by effective isotropic or anisotropic potentials. As a first application, we analyze the quantum states of two 3He atoms encapsulated in C60. At the Hartree–Fock level, we observe the breaking of spin and spatial symmetries. To ensure wavefunctions with the correct symmetries, we mix the broken-symmetry Hartree–Fock states within the non-orthogonal configuration interaction expansion. Our proposed approach predicts singly and triply degenerate ground states for the singlet (para-3He2@C60) and triplet (ortho-3He2@C60) nuclear spin configurations, respectively. The ortho-3He2@C60 ground state is 5.69 cm−1 higher in energy than the para-3He2@C60 ground state. The nuclear densities obtained for these states exhibit the icosahedral symmetry of the C60 embedding potential. Importantly, our calculated energies for the lowest 85 states are in close agreement with perturbation theory results based on a harmonic oscillator plus rigid rotor model of 3He2@C60.

Efficient Screening of Bi-Metallic Electrocatalysts for Glycerol Valorization

Glycerol is a byproduct of biodiesel production and, as such, it is of limited economic value. By means of electrooxidation, glycerol can be used as a feedstock for scalable hydrogen production, in addition to conversion to value-added products. The development of novel and efficient catalytic electrode materials for the anodic side of the reaction is a key towards a hydrogen-based energy economy. In the present study, a computational screening protocol combining DFT, scaling relations, and microkinetic modeling allows for a rational selection of novel catalysts that can deliver efficient glycerol electrooxidation, low cost of production, and environmental sustainability. Activity and chemical selectivity towards hydrogen production on pure metal catalysts is discussed in terms of volcano-shaped plots. We find that the selectivity in the glycerol oxidation reaction is influenced by a different energy landscape when in the presence of water and best classified by a comparison of O-H and C-H bond-breaking barriers. In addition, we screened 3570 bi-metallic catalysts in the AB (L1(0)) and A(3)B (L1(2)) ordered structures for activity, stability, price, and toxicity. By filtering based on the criteria for toxicity, resistance to oxidation, miscibility, and price, we have identified 5 L1(0) structured catalysts (AgPd, AuPd, PtSb, CuPt, and AgPt) and 20 L1(2) catalysts (Ga3Ta, In3Ta, Ir3W, Ir3Mo, Cu3Pt, Ir3Ta, Ir3Re, Pd3Bi, Pd3Cu, Pd3W, Pd3Co, Pd3Sn, Pd3Mo, Pd3Ag, Pd3Ga, Pd3Ta, Au3Ru, Pd3In, Au3Ir, and Pd3Au) that are all predicted to show high activity. We also identify an additional 37 L1(0) and 92 L1(2) structured electrocatalysts with an anticipated medium-high activity. (C) 2021 The Author(s). Published by Elsevier Ltd

Chemisorbed oxygen or surface oxides steer the selectivity in Pd electrocatalytic propene oxidation observed by operando Pd L-edge X-ray absorption spectroscopy

Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8-1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. These reactions are proposed to proceed via the Langmuir-Hinshelwood mechanism. In contrast, for the potential range of 1.1-1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface

X‑ray Scattering and O–O Pair-Distribution Functions of Amorphous Ices

The structure factor and oxygen–oxygen pair-distribution functions of amorphous ices at liquid nitrogen temperature (<i>T</i> = 77 K) have been derived from wide-angle X-ray scattering (WAXS) up to interatomic distances of <i>r</i> = 23 Å, where local structure differences between the amorphous ices can be seen for the entire range. The distances to the first coordination shell for low-, high-, and very-high-density amorphous ice (LDA, HDA, VHDA) were determined to be 2.75, 2.78, and 2.80 Å, respectively, with high accuracy due to measurements up to a large momentum transfer of 23 Å^–1. Similarities in pair-distribution functions between LDA and supercooled water at 254.1 K, HDA and liquid water at 365.9 K, and VHDA and high-pressure liquid water were found up to around 8 Å, but beyond that at longer distances, the similarities were lost. In addition, the structure of the high-density amorphous ices was compared to high-pressure crystalline ices IV, IX , and XII, and conclusions were drawn about the local ordering

Water: A Tale of Two Liquids

Water is the most abundant liquid on earth and also the substance with the largest number of anomalies in its properties. It is a prerequisite for life and as such a most important subject of current research in chemical physics and physical chemistry. In spite of its simplicity as a liquid, it has an enormously rich phase diagram where different types of ices, amorphous phases, and anomalies disclose a path that points to unique thermodynamics of its supercooled liquid state that still hides many unraveled secrets. In this review we describe the behavior of water in the regime from ambient conditions to the deeply supercooled region. The review describes simulations and experiments on this anomalous liquid. Several scenarios have been proposed to explain the anomalous properties that become strongly enhanced in the supercooled region. Among those, the second critical-point scenario has been investigated extensively, and at present most experimental evidence point to this scenario. Starting from very low temperatures, a coexistence line between a high-density amorphous phase and a low-density amorphous phase would continue in a coexistence line between a high-density and a low-density liquid phase terminating in a liquid-liquid critical point, LLCP. On approaching this LLCP from the one-phase region, a crossover in thermodynamics and dynamics can be found. This is discussed based on a picture of a temperature-dependent balance between a high-density liquid and a low-density liquid favored by, respectively, entropy and enthalpy, leading to a consistent picture of the thermodynamics of bulk water. Ice nucleation is also discussed, since this is what severely impedes experimental investigation of the vicinity of the proposed LLCP. Experimental investigation of stretched water, i.e., water at negative pressure, gives access to a different regime of the complex water diagram. Different ways to inhibit crystallization through confinement and aqueous solutions are discussed through results from experiments and simulations using the most sophisticated and advanced techniques. These findings represent tiles of a global picture that still needs to be completed. Some of the possible experimental lines of research that are essential to complete this picture are explored