GPAW: An open Python package for electronic structure calculations.

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.

Water in PHI Nanopores: Modeling Adsorption, Solvent Structure, and Thermodynamics.

We modeled the uptake of water molecules into the nanopores of potassium-polyheptazineimide (K-PHI), a 2D covalent material that is one of the best water-splitting photocatalysts to date possessing experimentally reported strong water binding. In the current models, we find that first water molecules are bound with -94.5 kJ/mol, i.e., twice the cohesion energy of water and one of the highest adsorption enthalpies reported so far. This strong binding proceeds unexpectedly on a similar enthalpy level until the pore is filled, while the binding strength is passed through a conjugated water network. The tight binding is also expressed in calculated, strongly shortened O-O distances, which are on average about 5% shorter than in bulk water, which corresponds to a much higher water density, for a 2D structure above 1.1 g/ cm3. The H-bridges are strongly aligned in the direction perpendicular to the covalent planes, which could give reasons for the experimentally observed ultrahigh ion fluxes and conductivity of K-PHI membranes. Decomposition of the adsorption energy into components reveals an unexpectedly high charge transfer contribution, where the partly naked K+ ions play a key role. The latter fact not only offers a new structural lead motif for the design of more strongly, but reversibly binding adsorption materials involving metal ions on their surface but also puts cations as known cofactors in enzymes into a new light.

Molybdenum Disulfide Nanoribbons with Enhanced Edge Nonlinear Response and Photoresponsivity.

MoS2 nanoribbons have attracted increased interest due to their properties, which can be tailored by tuning their dimensions. Herein, the growth of MoS2 nanoribbons and triangular crystals formed by the reaction between films of MoOx (2

Assignment of the Crystal Structure to the Aza-Pinacol Coupling Product by X-ray Diffraction and Density Functional Theory Modeling.

Aza-pinacol coupling of N-benzyl-1-phenylmethanimine using Zn dust affords a mixture of R,S- or R,R-diastereomers in a 1:1 ratio. The R,S-diastereomer is solid with an m.p. of 135 °C, while the R,R-diastereomer is liquid at room temperature. The configuration of stereocenters was determined by combining X-ray powder diffraction and density functional theory (DFT) modeling.

Real-Space Identification of Non-Noble Single Atomic Catalytic Sites within Metal-Coordinated Supramolecular Networks.

With regard to the development of single atom catalysts (SACs), non-noble metal-organic layers combine a large functional variability with cost efficiency. Here, we characterize reacted layers of melamine and melem molecules on a Cu(111) surface by noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and ab initio simulations. Upon deposition on the substrate and subsequent heat treatments in ultrahigh vacuum (UHV), these precursors undergo a stepwise dehydrogenation. After full dehydrogenation of the amino groups, the molecular units lie flat and are strongly chemisorbed on the copper substrate. We observe a particularly extreme interaction of the dehydrogenated nitrogen atoms with single copper atoms located at intermolecular sites. In agreement with the nc-AFM measurements performed with an O-terminated copper tip on these triazine- and heptazine-based copper nitride structures, our ab initio simulations confirm a pronounced interaction of oxygen species at these N-Cu-N sites. To investigate the related functional properties of our samples regarding the oxygen reduction reaction (ORR), we developed an electrochemical setup for cyclic voltammetry experiments performed at ambient pressure within a drop of electrolyte in a controlled O2 or N2 environment. Both copper nitride structures show a robust activity in irreversibly catalyzing the reduction of oxygen. The activity is assigned to the intermolecular N-Cu-N sites of the triazine- and heptazine-based copper nitrides or corresponding oxygenated versions (N-CuO-N, N-CuO2-N). By combining nc-AFM characterization on the atomic scale with a direct electrochemical proof of performance, our work provides fundamental insights about active sites in a technologically highly relevant reaction.

"Red Carbon": A Rediscovered Covalent Crystalline Semiconductor.

Carbon suboxide (C3 O2 ) is a unique molecule able to polymerize spontaneously into highly conjugated light-absorbing structures at temperatures as low as 0 °C. Despite obvious advantages, little is known about the nature and the functional properties of this carbonaceous material. In this work, the aim is to bring "red carbon," a forgotten polymeric semiconductor, back to the community's attention. A solution polymerization process is adapted to simplify the synthesis and control the structure. This allows one to obtain this crystalline covalent material at low temperatures. Both spectroscopic and elemental analyses support the chemical structure represented as conjugated ladder polypyrone ribbons. Density functional theory calculations suggest a crystalline structure of AB stacks of polypyrone ribbons and identify the material as a direct bandgap semiconductor with a medium bandgap that is further confirmed by optical analysis. The material shows promising photocatalytic performance using blue light. Moreover, the simple condensation-aromatization route described here allows the straightforward fabrication of conjugated ladder polymers and can be inspiring for the synthesis of carbonaceous materials at low temperatures in general.

Photocatalytic Water Splitting Reaction Catalyzed by Ion-Exchanged Salts of Potassium Poly(heptazine imide) 2D Materials.

Potassium poly (heptazine imide) (K-PHI), a crystalline two-dimensional carbon-nitride material, is an active photocatalyst for water splitting. The potassium ions in K-PHI can be exchanged with other ions to change the properties of the material and eventually to design the catalysts. We report here the electronic structures of several ion-exchanged salts of K-PHI (K, H, Au, Ru, and Mg) and their feasibility as water splitting photocatalysts, which were determined by density functional theory (DFT) calculations. The DFT results are complemented by experiments where the performances in the photocatalytic hydrogen evolution reaction (HER) were recorded. We show that due to its narrow band gap, Ru-PHI is not a suitable photocatalyst. The water oxidation potentials are straddled between the band edge potentials of H-PHI, Au-PHI, and Mg-PHI; thus, these are active photocatalysts for both the oxygen and hydrogen evolution reactions, whereas K-PHI is active only for the HER. The experimental data show that these are active HER photocatalysts, in agreement with the DFT results. Furthermore, Mg-PHI has shown remarkable performance in the HER, with a rate of 539 µmol/(h·g) and a quantum efficiency of 7.14% at 410 nm light irradiation, which could be due to activation of the water molecule upon adsorption, as predicted by our DFT calculations.

Electrochemical N2 Reduction to Ammonia Using Single Au/Fe Atoms Supported on Nitrogen-Doped Porous Carbon.

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3) is a promising alternative route for an NH3 synthesis at ambient conditions to the conventional high temperature and pressure Haber-Bosch process without the need for hydrogen gas. Single metal ions or atoms are attractive candidates for the catalytic activation of non-reactive nitrogen (N2), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N2 activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic C2N materials. Our results show that the metal atoms present in the structure of C2N are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N2. We further demonstrate that a lower onset electrode potential is required for Fe-C2N than for Au-C2N. Thus, Fe-C2N is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au-C2N owing to the larger adsorption energy of N2 molecules. Furthermore, we have experimentally shown that single sites of Au and Fe supported on nitrogen-doped porous carbon are indeed active NRR catalysts. However, in contrast to our theoretical results, the Au-based catalyst performed slightly better with a Faradaic efficiency (FE) of 10.1% than the Fe-based catalyst with an FE of 8.4% at -0.2 V vs. RHE. The DFT calculations suggest that this difference is due to the competitive hydrogen evolution reaction and higher desorption energy of ammonia.

On the Possibility of Helium Adsorption in Nitrogen Doped Graphitic Materials.

The potassium salt of polyheptazine imide (K-PHI) is a promising photocatalyst for various chemical reactions. From powder X-ray diffraction data an idealized structural model of K-PHI has been derived. Using atomic coordinates of this model we defined an energetically optimized K-PHI structure, in which the K ions are present in the pore and between the PHI-planes. The distance between the anion framework and K+ resembles a frustrated Lewis pair-like structure, which we denote as frustrated Coulomb pair that results in an interesting adsorption environment for otherwise non-adsorbing, non-polar gas molecules. We demonstrate that even helium (He) gas molecules, which are known to have the lowest boiling point and the lowest intermolecular interactions, can be adsorbed in this polarized environment with an adsorption energy of  - 4.6 kJ mol-1 per He atom. The interaction between He atoms and K-PHI is partially originating from charge transfer, as disclosed by our energy decomposition analysis based on absolutely localized molecular orbitals. Due to very small charge transfer interactions, He gas adsorption saturates at 8 at%, which however can be subject to further improvement by cation variation.

Opposing Electronic and Nuclear Quantum Effects on Hydrogen Bonds in H2 O and D2 O.

The effect of extending the O-H bond length(s) in water on the hydrogen-bonding strength has been investigated using static ab initio molecular orbital calculations. The "polar flattening" effect that causes a slight σ-hole to form on hydrogen atoms is strengthened when the bond is stretched, so that the σ-hole becomes more positive and hydrogen bonding stronger. In opposition to this electronic effect, path-integral ab initio molecular-dynamics simulations show that the nuclear quantum effect weakens the hydrogen bond in the water dimer. Thus, static electronic effects strengthen the hydrogen bond in H2 O relative to D2 O, whereas nuclear quantum effects weaken it. These quantum fluctuations are stronger for the water dimer than in bulk water.