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Moreover, they illustrate the wide range of available tools, including crystal structure prediction (CSP) methods,[[11]] high-throughput calculations,[[13]] and machine learning (ML),[[15], [17]] and the role that databases, such as AFLOW,[19] Materials Project,[20] the Open Quantum Materials Database,[21] the Computational Materials Repository,[22] and the International Crystallographic Structure Database,[23] among many others, play in materials discovery. Tremendous advances in first-principles program packages, spectacular speed-ups in computer hardware coupled with significant algorithmic developments in crystal structure prediction, high-throughput screenings, data-mining, machine learning (ML), and artificial intelligence have made the dream of computational materials discovery a reality.[1] Recent success stories of materials that were first predicted and later verified experimentally include high-temperature superconductors,[[2], [4]] high-entropy high-hardness materials,[5] Heusler intermetallics for thermoelectrics and spintronics,[6] organic light-emitting diodes,[7] Li battery cathode materials,[8] porous materials for gas storage,[9] and carbon-boron clathrates.[10] The broad range of materials-by-design success stories with applications, including energy, quantum, structural, 2D materials, molecular crystals, molecular organic frameworks, and so on, have inspired us to devote this issue of JCP toward recent advances in this field. Parsaeifard I et al. i present a method to calculate fingerprint vectors from fingerprint distances, which measure the similarity of two structures.[27] This method is shown to automatically detect grain boundaries and the edges of carbon flakes without human input. [Extracted from the article]

Over the past decade, a combination of crystal structure prediction techniques and experimental synthetic work has thoroughly explored the phase diagrams of binary hydrides under pressure. The fruitfulness of this dual approach is demonstrated in the recent identification of several superconducting hydrides with T c s approaching room temperature. We start with an overview of the computational procedures for predicting stable structures and estimating their propensity for superconductivity. A survey of phases with high T c reveals some common structural features that appear conducive to the strong coupling of the electronic structure with atomic vibrations that leads to superconductivity. We discuss the stability and superconducting properties of phases containing two of these—molecular H 2 units mixed with atomic H and hydrogenic clathrate-like cages—as well as more unique motifs. Finally, we argue that ternary hydride phases, whose exploration is still in its infancy, are a promising route to achieve simultaneous superconductivity at high temperatures and stability at low pressures. Several ternary hydrides arise from the addition of a third element to a known binary hydride structure through site mixing or onto a new site, and several more are based on altogether new structural motifs. [ABSTRACT FROM AUTHOR]

The theoretical exploration of the phase diagrams of binary hydrides under pressure using ab initio crystal structure prediction techniques coupled with first principles calculations has led to the in silico discovery of numerous novel superconducting materials. This Perspective focuses on the alkaline earth and rare earth polyhydrides whose superconducting critical temperature, Tc, was predicted to be above the boiling point of liquid nitrogen. After providing a brief overview of the computational protocol used to predict the structures of stable and metastable hydrides under pressure, we outline the equations that can be employed to estimate Tc. The systems with a high Tc can be classified according to the motifs found in their hydrogenic lattices. The highest Tcs are found for cages that are reminiscent of clathrates and the lowest for systems that contain atomic and molecular hydrogen. A wide variety of hydrogenic motifs including 1- and 2-dimensional lattices, as well as H 10 δ − molecular units comprising fused H 5 δ − pentagons, are present in phases with intermediate Tcs. Some of these phases are predicted to be superconducting at room temperature. Some may have recently been synthesized in diamond anvil cells. [ABSTRACT FROM AUTHOR]

The high critical superconducting temperatures (Tcs) of metal hydride phases with clathrate‐like hydrogen networks have generated great interest. Herein, we employ the Density Functional Theory‐Chemical Pressure (DFT‐CP) method to explain why certain electropositive elements adopt these structure types, whereas others distort the hydrogenic lattice, thereby decreasing the Tc. The progressive opening of the H24 polyhedra in MH6 phases is shown to arise from internal pressures exerted by large metal atoms, some of which may favor an even higher hydrogen content that loosens the metal atom coordination environments. The stability of the LaH10 and LaBH8 phases is tied to stuffing of their shared hydrogen network with either additional hydrogen or boron atoms. The predictive capabilities of DFT‐CP are finally applied to the Y−X−H system to identify possible ternary additions yielding a superconducting phase stable to low pressures. [ABSTRACT FROM AUTHOR]

The high critical superconducting temperatures (Tcs) of metal hydride phases with clathrate‐like hydrogen networks have generated great interest. Herein, we employ the Density Functional Theory‐Chemical Pressure (DFT‐CP) method to explain why certain electropositive elements adopt these structure types, whereas others distort the hydrogenic lattice, thereby decreasing the Tc. The progressive opening of the H24 polyhedra in MH6 phases is shown to arise from internal pressures exerted by large metal atoms, some of which may favor an even higher hydrogen content that loosens the metal atom coordination environments. The stability of the LaH10 and LaBH8 phases is tied to stuffing of their shared hydrogen network with either additional hydrogen or boron atoms. The predictive capabilities of DFT‐CP are finally applied to the Y−X−H system to identify possible ternary additions yielding a superconducting phase stable to low pressures. [ABSTRACT FROM AUTHOR]

Tetrahydrides crystallizing in the ThCr2Si2 structure type have been predicted to become stable for a plethora of metals under pressure, and some have recently been synthesized. Through detailed first‐principles investigations we show that the metal atoms within these I4/mmm symmetry MH4 compounds may be divalent, trivalent or tetravalent. The valence of the metal atom and its radius govern the bonding and electronic structure of these phases, and their evolution under pressure. The factors important for enhancing superconductivity include a large number of hydrogenic states at the Fermi level, and the presence of quasi‐molecular H2δ- units whose bonds have been stretched and weakened (but not broken) via electron transfer from the electropositive metal, and via a Kubas‐like interaction with the metal. Analysis of the microscopic mechanism of superconductivity in MgH4, ScH4 and ZrH4 reveals that phonon modes involving a coupled libration and stretch of the H2δ- units leading to the formation of more complex hydrogenic motifs are important contributors towards the electron phonon coupling mechanism. In the divalent hydride MgH4, modes associated with motions of the hydridic hydrogen atoms are also key contributors, and soften substantially at lower pressures. [ABSTRACT FROM AUTHOR]

We report a microhardness indentation study for multicomponent refractory metal boride thin films that belong to the family of high‐entropy ceramics exhibiting superior hardness and high temperature properties. We focus on the nominally equimolar composition (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2, which we refer to as HEB3, on c‐plane sapphire substrates. Thin films are prepared using bipolar high power impulse magnetron sputtering (HiPIMS), where the positive kick pulse is optimized to produce films with high density, high crystallinity, and a microstructure that is isometric in nature as revealed by cross‐sectional fracture surface imaging, X‐ray diffraction, and X‐ray reflectometry. Low‐load Knoop indentation is used to measure microhardness. We find strong trends linking kick‐pulse magnitude, microstructure uniformity, high density, and Knoop hardness. Optimized conditions can produce films with low‐load Knoop hardness exceeding 30 GPa. To validate these trends, we present a rigorous indent characterization campaign and discussion considering possible artifacts from roughness, thickness, and indent loading. The key finding of our study is the ability of engineering bombardment by using a low duty‐cycle pulsed plasma to manipulate crystal structure without sacrificing crystallinity, and in turn boost material hardness. [ABSTRACT FROM AUTHOR]

First-principles calculations coupled with evolutionary algorithms have been employed to predict the structures of hydrides of the alkali metals and alkaline earth metals under pressure. A plethora of novel phases with MHand M=Li, Na, K, Rb, Cs, and MHand M=Mg, Ca, Sr, Ba have been predicted to be stable at pressures that can be achieved in a diamond anvil cell. A number of structural motifs, including HHHone-dimensional hydrogenic chains or three-dimensional sodalite-like lattices of hydrogen atoms, are found in the stable phases. The electronic structure and mechanism of metalization under pressure turn out to depend on the hydrogenic motifs that are present. Phases with 1D or 3D hydrogenic lattices are predicted to be the best candidates for high-temperature superconductivity, with those containing Hmolecules following suit. Structures with H–or Hare not predicted to be good superconductors. Chemical trends regarding the pressure required for stabilization of the polyhydrides are discussed. Experimental studies that have successfully synthesized the polyhydrides of lithium and sodium are described. Lithium subhydrides, Liwith, are also predicted to be stable between 50–100 GPa. [ABSTRACT FROM PUBLISHER]

The article presents a research which reveals that the alkali metal polyhydrides becomes stable after its decomposition. Information regarding the preparation of intriguing systems which exhibits strong intermolecular interactions under pressure including the van der Waals is presented. It mentions that subhydrides are composed of superalkali building blocks and exhibits ionic bonding on the inside of metallic bonding in the interstitial regions.

The optical absorption spectra of alkali metals in ethylenediamine have provided evidence for a third oxidation state, -1, of all of the alkali metals heavier than lithium. Experimentally determined NMR parameters have supported this interpretation, further indicating that whereas Na- is a genuine metal anion, the interaction of the alkali anion with the medium becomes progressively stronger for the larger metals. Herein, first-principles computations based upon density functional theory are carried out on various species which may be present in solutions composed of alkali metals and ethylenediamine. The energies of a number of hypothetical reactions computed with a continuum solvation model indicate that neither free metal anions, M-, nor solvated electrons are the most stable species. Instead, [Li(en)3]2 and [M(en)3δ+·Mδ-] (M = Na, K, Rb, Cs) are predicted to have enhanced stability. The M(en)3 complexes can be viewed as superalkalis or expanded alkalis, ones in which the valence electron density is pulled out to a greater extent than in the alkali metals alone. The computed optical absorption spectra and NMR parameters of the [Li(en)3]2 superalkali dimer and the [M(en)3δ+·Mδ-] superalkali-alkali mixed dimers are in good agreement with the aforementioned experimental results, providing further evidence that these may be the dominant species in solution. The latter can also be thought of as an ion pair formed from an alkali metal anion (M-) and solvated cation (M(en)3+). [ABSTRACT FROM AUTHOR]

Using the newly developed NMTO (muffin-tin orbilals of order technique, which generates atom-centered, localized, Wannier-like functions, we investigate whether - such as in graphiles - an intedayer band exists at the Fermi level in ThSi2 with six electrons per Si. We find that the Th s-like NMTO, not only in the graphite-structured β-ThSi2 but also in the twisted allotrope, α-ThSi2, strikingly resembles the previously calculated inlerlayer NMTO in graphiles and that the corresponding band cuts the Fermi level but is at many places gapped, particularly in the twisted allotrope. This suggests that the mechanism of superconductivity in both allotropes of ThSi2 is similar to the one in intercalated graphites such as CaC6 with 41/3 electrons per C. Here, C buckling and Ca in-plane phonons couple between intetlayer and π* bands. In ThSi2, not only these bands but also the a band cross the Fermi level, and intraband coupling via Si bond-stretching modes, like in MgB2, is likely to contribute to the superconductivily. Finally, we investigate the bonding in ThSi2 by seeking those NMTO basis sets with merely nine or six orbitals per ThSi2, which best span the occupied bands. Our results indicate that in both allotropes there are three weakened Si-Si σ bonds, tWO Th-Si spz bonds, and a nonbonding Si p2-orbital. [ABSTRACT FROM AUTHOR]

NMR (nuclear magnetic resonance) is a versatile experimental tool to study the properties of molecules and solids. It has been proposed that reliable computational data of the 13C NMR chemical shifts of different types of carbon nanotubes may be used to guide experimental characterization by NMR. Within the last few years this field has become quite active. After outlining the background for first-principles calculations, as well as early model calculations, we focus on recent first-principles theoretical studies performed toward this end. Studies on finite and infinite SWNT systems have indicated that 13C NMR may be used to determine the diameter distribution of the tubes in a bulk sample. The Knight shift of metallic tubes has been examined. NICS (nucleus independent chemical shifts) have yielded information about the aromaticity of various systems, and the NMR chemical shifts of small molecules trapped in nanotubes have been calculated. Work on SWNTs functionalized with NR groups has suggested that 13C NMR may be used to determine which nanotube carbons are derivatized, and perhaps even yield information about the diameter of the tubes. It has also been found that 13C NMR may be useful to quantify the degree of fluorination. Studies on Stone–Wales defects have indicated that well-resolved NMR signals may arise from atoms in the defect site. Shielding tensor data is also discussed. The theoretical progress made in this field shows that a wealth of information is contained within the NMR chemical shifts of carbon nanotubes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

From detailed assessments of electronic structure, we find that a combination of significantly quantal elements, six of seven atoms being hydrogen, becomes a stable metal at a pressure approximately 1/4 of that required to metalize pure hydrogen itself. The system, LiH6 (and other LiHn), may well have extensions beyond the constituent lithium. These hypothetical materials demonstrate that nontraditional stoichiometries can considerably expand the view of chemical combination under moderate pressure. [ABSTRACT FROM AUTHOR]

The 13C NMR chemical shifts for functionalized (7,0), (8,0), (9,0), and (10,0) single-walled carbon nanotubes (SWNTs) have been studied computationally using gauge-including projector-augmented plane-wave (GIPAW) density functional theory (DFT). The functional groups NH, NCH3, NCH2OH, and CH2NHCH2 have been considered, and different sites where covalent addition or substitution may occur have been examined. The shifts of the carbons directly attached to the group are sensitive to the bond which has been functionalized and may, therefore, be used to identify whether the group has reacted with a parallel or a diagonal CC bond. The addition of NH to a parallel bond renders the functionalized carbons formally sp³-hybridized, yielding shifts of around 44 ppm, independent of the SWNT radius. Reaction with a diagonal bond retains the formal sp² hybridization of the substituted carbons, and their shifts are slightly lower or higher than those of the unsubstituted carbon atoms. The calculated 1H NMR shifts of protons in the functional groups are also dependent upon the SWNT-group interaction. Upon decreasing the degree of functionalization for the systems where the group is added to a parallel bond, the average chemical shift of the unfunctionalized carbons approaches that of the pristine tube. At the same time, the shifts of the functionalized carbons remain independent upon the degree of functionalization. For the SWNTs where N-R attaches to a parallel bond, the average shift of the sp² carbons was found to be insensitive to the substituent R. Moreover, the shifts of the functionalized sp³ carbons, as well as of the carbons within the group itself, are independent of the SWNT radius. The results indicate that a wealth of knowledge may be obtained from the 13C NMR of functionalized SWNTs. [ABSTRACT FROM AUTHOR]

The electronic structure and 13C NMR chemical shift of (9,0) single-walled carbon nanotubes (SWNTs) are investigated theoretically. Shielding tensor components are also reported. Density functional calculations were carried out for C30-capped and H-capped fragments which serve as model systems for the infinite (9,0) SWNT. Based on the vanishing HOMO-LUMO gap, H-capped nanotube fragments are predicted to exhibit "metallic" behavior. The 13C chemical shift approaches a value of ≈1 33 ppm for the longest fragment studied here. The C30-capped SWNT fragments of D3dD3h symmetry, on the other hand, are predicted to be small-gap semiconductors just like the infinite (9,0) SWNT. The differences in successive HOMO-LUMO gaps and HOMO and LUMO energies, as well as the 13C NMR chemical shifts, converge slightly faster with the fragment's length than for the H-capped tubes. The difference between the H-capped and C30-capped fragments is analyzed in some detail. The results indicate that (at least at lengths currently accessible to quantum chemical computations) the H-capped systems represent less suitable models for the (9,0) SWNT because of pronounced artifacts due to their finite length. From our calculations for the C30-capped fragments, the chemical shift of a carbon atom in the (9,0) SWNT is predicted to be about 130 ppm. This value is in reasonably good agreement with experimental estimates for the 13C chemical shift in SWNTs. [ABSTRACT FROM AUTHOR]

Single-site homogeneous catalysts need to be activated by a co-catalyst or counterion. The high activity imparted by methylaluminoxane (MAO) has caused it to be one of the most important co-catalysts. In fact it can be argued that the success of the metallocenes is largely due to the discovery of MAO. However, despite intensive studies MAO has remained a ‘black box’. The presence of multiple equilibria between different (AlOMe)n oligomers coupled with the interaction between MAO and TMA has hindered experimental structural assignment of MAO. This has made it nearly impossible to characterize the dormant and active species present in olefin polymerization and therefore to theoretically investigate the mechanism of this process. Moreover, the binding of MAO with porous inorganic oxides such as silica, alumina and MgCl2 is currently not understood. Perhaps, even more puzzling is the fact that a large excess of MAO is necessary in order for polymerization to occur (Al:catalyst ratios of ∼10,000:1), whereas in the case of supported MAO this ratio is greatly decreased (ratios of ∼100:1–500:1). Despite the fact that the co-catalytic ability of MAO was discovered nearly 25 years ago, its exact structure and function is still unknown. In recent years, theoretical studies of MAO aiming to give further insight into the aforementioned issues have emerged. In this article, we give a quick overview of experimental studies of MAO and an in-depth review of recent theoretical investigations. [Copyright &y& Elsevier]

Density functional calculations, electronic structure, high pressure chemistry, solid-state structures, superconductors Keywords: density functional calculations; electronic structure; high pressure chemistry; solid-state structures; superconductors EN density functional calculations electronic structure high pressure chemistry solid-state structures superconductors 14788 14788 1 10/29/21 20211025 NES 211025 B Exotic physical properties b are often exhibited by compounds crystallizing in the ubiquitous ThCr SB 2 sb Si SB 2 sb structure type illustrated in the picture. [Extracted from the article]

The X2MH6 family, consisting of an electropositive cation Xn+ and a main group metal M octahedrally coordinated by hydrogen, have been identified as promising templates for high‐temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the anions IrH64− anions are key for the superconducting mechanism, and they induce coupling in the eg* ${e_g^{\ast} }$ set of orbitals, which are antibonding between the H 1s and the Ir dx2-y2 ${d_{x^2 - y^2 } }$ or dz2 ${d_{z^2 } }$ orbitals. Because calcium possesses low‐lying d‐orbitals, eg* ${e_g^{\ast} }$ →Ca d back‐donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may provide rules for identifying likely high‐temperature superconductors in other systems where the antibonding anionic states are filled. [ABSTRACT FROM AUTHOR]

At high pressure, the typical behavior of elements dictated by the periodic table—including oxidation numbers, stoichiometries in compounds, and reactivity, to name but a few—is altered dramatically. As pressure is applied, the energetic ordering of atomic orbitals shifts, allowing core orbitals to become chemically active, atypical electron configurations to occur, and in some cases, non-atom-centered orbitals to form in the interstices of solid structures. Strange stoichiometries, structures, and bonding motifs result. Crystal structure prediction tools, not burdened by preconceived notions about structural chemistry learned at atmospheric pressure, have been applied to great success to explore phase diagrams at high pressure, identifying novel structures in diverse chemical systems. Several of these phases have been subsequently synthesized. Experimentally, access to high-pressure regimes has been bolstered by advances in diamond anvil cell and dynamic compression techniques. The joint efforts of experiment and theory have led to startling successes stories in the realm of high-temperature superconductivity, identifying many novel phases—some of which have been synthesized—whose superconducting transition approaches room temperature. [ABSTRACT FROM AUTHOR]

Super‐hard (Hf,Mo,Ti,V,W)B2 was synthesized by boro‐carbothermal reduction and densified by spark plasma sintering. This composition was produced for the first time as a single‐phase ceramic in the present research. The optimized ceramic had a single hexagonal AlB2‐type crystalline phase with a grain size of 3.8 µm and homogeneous distribution of the constituent metals. The Vickers hardness exhibited the indentation size effect, increasing from 27 GPa at a load of 9.8 N to as high as 66 GPa at a load of 0.49 N. This is the highest hardness reported to date for high entropy boride ceramics. [ABSTRACT FROM AUTHOR]

The silver‐fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3F4 and Ag2F3) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag2F5, containing both Ag2+ and Ag3+, we find that strong 1D antiferromagnetic coupling is retained throughout the pressure‐induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed‐valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed‐valence compounds of silver at high pressure. [ABSTRACT FROM AUTHOR]

The heavier alkali metal hydrides MH (M = K, Rb, Cs) undergo a series of pressure induced structural phase transitions: B1 (NaCl) → B2 (CsCl) → CrB. Experiments reveal that the latter occurs at 85 and 17.5 GPa for RbH and CsH, but it has not yet been observed for KH. Herein, evolutionary algorithms coupled with density functional theory calculations are employed to explore the potential energy surface of the aforementioned hydrides up to pressures of 300 GPa. The computations support previous theoretical work which predicts that KH will adopt the CrB structure when compressed. In addition, for KH and RbH we find configurations with Pnma and I41/amd symmetry that are thermodynamically competitive with the CrB structure at 300 GPa. Between 100-150 GPa, a Pnma structure which is analogous to a high-pressure form of CsI is found to be the most stable phase for the heaviest alkali hydride considered. At higher pressures a hitherto unknown CsH-P63/mmc arrangement becomes thermodynamically preferred up to at least 400 GPa. A detailed analysis of the geometric and electronic structures of the various phases is provided. [ABSTRACT FROM AUTHOR]

First-principles calculations and analysis of the thermodynamic, structural, and electronic properties of liquid SiO 2 characterize the bonded-to-atomic transition at 0.1–1.6 TPa and 10 4 – 10 5 K (1–9 eV), the high-energy-density regime relevant to understanding planetary interiors. We find strong ionic bonds that become short-lived due to high kinetics during the transition, with sensitivity of the transition temperature to pressure, and our calculated Hugoniots agree with past experimental data. These results reconcile previous experimental and theoretical findings by clarifying the nature of the bond dissociation process in early Earth and "rocky" (oxide) constituents of large planets. [ABSTRACT FROM AUTHOR]

The surface functionalization of silica particles for chromatographic applications is typically achieved by means of silanization reactions. A silane molecule (e.g., chlorosilanes or alkoxysilanes) with a functional group of interest is attached to the silica surface through a siloxane bond. Even though the surface modification via silanization is widely used, how the reaction parameters affect the degree of surface coverage is rarely discussed, particularly when using the less reactive alkoxysilanes reagents. We studied reaction conditions to graft p -aminophenyltrimethoxysilane (p-APTMS) on superficially porous particles. Theoretical calculations predicted that 3 grafts of p-APTMS per nm2 (~5 μmol/m2) can be accommodated on the silica surface. This value is not achieved unless the reaction conditions are optimized. To maximize the surface coverage of the aminophenyl layer at the silica surface, we investigated the influence of temperature, reaction time, and addition of water to the silanization reaction on the surface coverage. After optimization, it was found that using elevated temperatures (130 °C), adding 3.3 equivalents of water per p-APTMS molecule, and reacting for 24 h in decane as the solvent provided a surface coverage as high as 4.5 μmol/m2 (2.7 grafts/nm2), approaching the limit suggested via theoretical density functional theory calculations. • The reaction of p -aminophenyl trimethoxysilane (p-APTMS) on superficially porous silica particles was studied. • Reaction temperature, time, and water content in the reaction were optimized to maximized surface coverage. • Computational calculations predicted a p-APTMS surface coverage of approximately 5 μmol/m2. • Optimized reaction conditions resulted in a surface coverage of ~4.5 μmol/m2. [ABSTRACT FROM AUTHOR]

Abstract Silicon (Si) is one of the most essential elements, as it is indispensable for modern electronic technology. The standard Si structure at ambient conditions is the cubic diamond structure, and it has an indirect band gap, which prevents it from being considered as a next-generation platform for semiconductor technologies. Therefore, the search for new allotropes of silicon has attracted great attention. Herein, first principles swarm-intelligence structure searches coupled with density-functional theory were performed to explore the stable high-pressure phases of silicon-rich lithium containing compounds, LiSi x (x = 4 – 8). The LiSi 4 stoichiometry was predicted to be stable, and it was found to assume one of the following space groups, P 4/ mnc , Cmmm , and C 2/ m within the pressure range of 0 to 50 GPa. By removing the Li atoms from these compounds, three silicon allotropes were obtained that were metastable at ambient pressures. Our work illustrates how novel silicon allotropes can be predicted using the CALYPSO method. Highlights • This work systematically explored the high-pressure crystal structures of silicon-rich LiSi x (x = 4 – 8) in theory. • LiSi 4 was determined to undergo P 4 / mnc → Cmmm → C 2 / m phase transitions in the pressure range of 0–50 GPa. • By removing the Li atoms from the compounds, three silicon allotropes were obtained. • Our work provides new insights for exploring the silicon allotropes via CALYPSO method. [ABSTRACT FROM AUTHOR]

A picture of a chemical reaction of Lithium Subhydrides under Pressure and their Superatoms are presented.

Early quantum mechanical models suggested that pressure drives solids towards free‐electron metal behavior where the ions are locked into simple close‐packed structures. The prediction and subsequent discovery of high‐pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na‐hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na‐pd hybrid orbitals point towards the center of an 11‐membered penta‐capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na‐hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure. [ABSTRACT FROM AUTHOR]

Early quantum mechanical models suggested that pressure drives solids towards free‐electron metal behavior where the ions are locked into simple close‐packed structures. The prediction and subsequent discovery of high‐pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na‐hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na‐pd hybrid orbitals point towards the center of an 11‐membered penta‐capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na‐hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure. [ABSTRACT FROM AUTHOR]

Evolutionary crystal structure prediction searches have been employed to explore the ternary Li–F–H system at 300 GPa. Metastable phases were uncovered within the static lattice approximation, with LiF3H2, LiF2H, Li3F4H, LiF4H4, Li2F3H, and LiF3H lying within 50 meV/atom of the 0 K convex hull. All of these phases contain H n F n + 1 − (n = 1, 2) anions and Li+ cations. Other structural motifs such as LiF slabs, H 3 + molecules, and Fδ− ions are present in some of the low enthalpy Li–F–H structures. The bonding within the H n F n + 1 − molecules, which may be bent or linear, symmetric or asymmetric, is analyzed. The five phases closest to the hull are insulators, while LiF3H is metallic and predicted to have a vanishingly small superconducting critical temperature. Li3F4H is predicted to be stable at zero pressure. This study lays the foundation for future investigations of the role of temperature and anharmonicity on the stability and properties of compounds and alloys in the Li–F–H ternary system. [ABSTRACT FROM AUTHOR]

Review: 209 refs. [ABSTRACT FROM AUTHOR]

Density functional theory calculations have been carried out to analyze the factors contributing to the stabilities of a set of recently synthesized quaternary polar intermetallic compounds, (Eu xM1- x)2Ge2Pb with M = Ca, Sr, and Ba. Experiments showed that these preferentially crystallized with Pbam (M = Ca) or Cmmm (M = Sr, Ba) symmetry. We systematically explored how the electronic energies of these structures depended on how they were 'colored' by the europium/M ions for a wide composition range. It was found that whereas there was very little site preference in the Cmmm structure, the 'B' site in the Pbam structure strongly preferred smaller cations. The configurational entropy was also found to play a role in determining which structures might be preferred. However, the experimentally obtained product ratios could only be fully rationalized by the Gibbs free energies of structures containing M:Eu ratios resembling those that were synthesized experimentally. Our results highlight the importance of calculating vibrational contributions to the entropy for realistic structure models (in terms of coloring and composition) to explain product ratios for syntheses carried out at high temperatures. [ABSTRACT FROM AUTHOR]

Cooperativity plays a critical role in self‐assembly and molecular recognition. A rigid aromatic oligoamide macrocycle with a cyclodirectional backbone binds with DABCO‐based cationic guests in a 2 : 1 ratio in high affinities (Ktotal ≈1013 M−2) in the highly polar DMF. The host–guest binding also exhibits exceptionally strong positive cooperativity quantified by interaction factors α that are among the largest for synthetic host–guest systems. The unusually strong positive cooperativity, revealed by isothermal titration calorimetry (ITC) and fully corroborated by mass spectrometry, NMR and computational studies, is driven by guest‐induced stacking of the macrocycles and stabilization from the alkyl end chains of the guests, interactions that appear upon binding the second macrocycle. With its tight binding driven by extraordinary positive cooperativity, this host–guest system provides a tunable platform for studying molecular interactions and for constructing stable supramolecular assemblies. [ABSTRACT FROM AUTHOR]

Cooperativity plays a critical role in self‐assembly and molecular recognition. A rigid aromatic oligoamide macrocycle with a cyclodirectional backbone binds with DABCO‐based cationic guests in a 2 : 1 ratio in high affinities (Ktotal ≈1013 M−2) in the highly polar DMF. The host–guest binding also exhibits exceptionally strong positive cooperativity quantified by interaction factors α that are among the largest for synthetic host–guest systems. The unusually strong positive cooperativity, revealed by isothermal titration calorimetry (ITC) and fully corroborated by mass spectrometry, NMR and computational studies, is driven by guest‐induced stacking of the macrocycles and stabilization from the alkyl end chains of the guests, interactions that appear upon binding the second macrocycle. With its tight binding driven by extraordinary positive cooperativity, this host–guest system provides a tunable platform for studying molecular interactions and for constructing stable supramolecular assemblies. [ABSTRACT FROM AUTHOR]

A metallic, covalently bonded carbon allotrope is predicted via first principles calculations. It is composed of an sp3 carbon framework that acts as a diamond anvil cell by constraining the distance between parallel cis‐polyacetylene chains. The distance between these sp2 carbon atoms renders the phase metallic, and yields two well‐nested nearly parallel bands that cross the Fermi level. Calculations show this phase is a conventional superconductor, with the motions of the sp2 carbons being key contributors to the electron–phonon coupling. The sp3 carbon atoms impart superior mechanical properties, with a predicted Vickers hardness of 48 GPa. This phase, metastable at ambient conditions, could be made by on‐surface polymerization of graphene nanoribbons, followed by pressurization of the resulting 2D sheets. A family of multifunctional materials with tunable superconducting and mechanical properties could be derived from this phase by varying the sp2 versus sp3 carbon content, and by doping. [ABSTRACT FROM AUTHOR]

A metallic, covalently bonded carbon allotrope is predicted via first principles calculations. It is composed of an sp3 carbon framework that acts as a diamond anvil cell by constraining the distance between parallel cis‐polyacetylene chains. The distance between these sp2 carbon atoms renders the phase metallic, and yields two well‐nested nearly parallel bands that cross the Fermi level. Calculations show this phase is a conventional superconductor, with the motions of the sp2 carbons being key contributors to the electron–phonon coupling. The sp3 carbon atoms impart superior mechanical properties, with a predicted Vickers hardness of 48 GPa. This phase, metastable at ambient conditions, could be made by on‐surface polymerization of graphene nanoribbons, followed by pressurization of the resulting 2D sheets. A family of multifunctional materials with tunable superconducting and mechanical properties could be derived from this phase by varying the sp2 versus sp3 carbon content, and by doping. [ABSTRACT FROM AUTHOR]

Species likely to be present in methylaluminoxane(MAO) are studiedvia dispersion-corrected DFT, which we show is able to accuratelypredict thermochemical parameters for the dimerization of trimethylaluminum(TMA). Both cage-like, (AlOMe)n,c, and TMA-bound nanotubes, (AlOMe)n,t·(AlMe3)m, are found to be important components of MAO. The most stable structureshave aluminum/oxygen atoms in environments whose average hybridizationapproaches sp3/sp2. The (AlOMe)n,t·(AlMe3)misomers with the lowest free energies possess Al−μ-Me–Albonds. At 298 K a novel Td-(AlOMe)16,coligomer is one of the most stable structuresamong the six stoichiometries with the lowest free energies: (AlOMe)20,c·(AlMe3)2, Td-(AlOMe)16,c, (AlOMe)18,c, (AlOMe)20,c·(AlMe3), (AlOMe)10,t·(AlMe3)4, and (AlOMe)20,c. As the temperature rises, the abundanceof (AlOMe)n,t·(AlMe3)mdecreases, and that of (AlOMe)n,cincreases. Because theformer are expected to be precursors for the active species in polymerization,this may in part be the reason why the cocatalytic activity of MAOdecreases at higher temperatures. [ABSTRACT FROM AUTHOR]

A computational experiment investigating the 1H and 13C nuclear magnetic resonance (NMR) chemical shifts of molecules with unpaired electrons has been developed and implemented. This experiment is appropriate for an upper-level undergraduate laboratory course in computational, physical, or inorganic chemistry. The chemical shift range for paramagnetic systems differs substantially from the well-known range of diamagnetic compounds. Students carried out density functional theory calculations of the chemical shifts of an organic radical and a related closed-shell system. This simple exercise illustrated that a single unpaired electron may result in dramatically different chemical shifts. Organometallic systems were also considered. The chemical shifts of the closed shell molecule ferrocene were compared to those of vanadocene and nickelocene, which afford three and two unpaired electrons, respectively. A natural bonding orbital (NBO) analysis was employed to study the electronic structure of NiCp2. [ABSTRACT FROM AUTHOR]

We have developed and implemented a computational experiment that explores the behavior of a [2]catenane as a molecular switch. The experiment introduces mechanically interlocked molecules and discusses their potential as artificial molecular machines. The relaxation rate and lifetime, as well as the ratio of the ground state and the metastable state configurations, are calculated in the gas phase as well as solvents of varying polarity using semiempirical quantum chemical calculations. The π-π donor-acceptor interactions between the two macrocycles comprising the catenane are probed via Hartree--Fock calculations of the frontier molecular orbitals. It is shown that NMR calculations may be employed to aid in the identification of various chemical species. The graphical user interface of the open-source molecular editor Avogadro was employed to visualize the crystal structure of the catenane, followed by quantum chemical calculations using the WebMO graphical interface. [ABSTRACT FROM AUTHOR]

A computational experiment that investigates the optical activity of the amino acid valine has been developed for an upper-level undergraduate physical chemistry laboratory course. Hybrid density functional theory calculations were carried out for valine to confirm the rule that adding a strong acid to a solution of an amino acid in the L configuration renders the optical rotation more positive. Correspondingly, if the optical rotation becomes more negative, the amino acid is of the D configuration. The students employed the open-source molecular editor Avogadro to build the molecules, conduct conformer searches, and calculate the energies of the conformers with a molecular mechanics force field. Subsequent geometry optimizations and optical rotation calculations were performed with a quantum chemistry program, using the WebMO graphical interface. The role of the solvent in stabilizing the zwitterionic form of an amino acid was investigated. [ABSTRACT FROM AUTHOR]

The cesium polyhydrides (CsHn, n > 1) are predicted to become stable, with respect to decomposition into CsH and H2, at pressures as low as 2 GPa. The CsH3 stoichiometry is found to have the lowest enthalpy of formation from CsH and H2 between 30 and 200 GPa. Evolutionary algorithms predict five distinct, mechanically stable, nearly isoenthalpic CsH3 phases consisting of H3 molecules and Cs+ atoms. The H-3 sublattices in two of these adopt a hexagonal three-connected net; in the other three the net is twisted, like the silicon sublattice in the α-ThSi2 structure. The former emerge as being metallic below 100 GPa in our screened hybrid density functional theory calculations, whereas the latter remain insulating up to pressures greater than 250 GPa. The Cs+ cations in the most-stable 141/amd CsH3 phase adopt the positions of the Cs atoms in Cs-IV, and the H3 molecules are found in the (interstitial) regions which display a maximum in the electron density. [ABSTRACT FROM AUTHOR]

Reported are the syntheses and the crystallographic characterization of two structurally related solid-state compounds: (Eu1-xCax)2Ge2Pb (space group Pbam) and (SrxEu1-x)2Ge2Pb (space group Cmmm). Both structures boast anionic sublattices with fully ordered Ge and Pb at the atomic level, which is unusual for elements of the same group. Despite the nearly identical formulas and the similar chemical makeup, the nature of the chemical bonding in the two compounds is subtly different; in the (Eu1-xCax)2Ge2Pb structure, Ge and Pb are positioned at a relatively shorter distance from one another (<3.0 Å). The close proximity of the atoms leads to interactions, which are seen for the first time in an extended structure and can be suggested to have a covalent character. This conjecture is supported by extensive electronic band-structure calculations using first principles. Magnetic susceptibility measurements reveal Eu2+ ground state ([Xe]4f configuration) and the presence of an antiferromagnetic ordering at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

New aromatic oligoamide macrocycles with C3-symmetry bind a bipyridinium guest (G) to form compact pseudo[3]rotaxanes involving interesting enthalpic and entropic contributions. The observed high stabilities and strong positive binding cooperativity are found in few other host–guest systems. [ABSTRACT FROM AUTHOR]