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Abstract The quasiharmonic approximation (QHA) in combination with density-functional theory is the main computational method used to calculate thermodynamic properties under arbitrary temperature and pressure conditions. QHA can predict thermodynamic phase diagrams, elastic properties and temperature- and pressure-dependent equilibrium geometries, all of which are important in various fields of knowledge. The main drawbacks of QHA are that it makes spurious predictions for the volume and other properties in the high temperature limit due to its approximate treatment of anharmonicity, and that it is unable to model dynamically stabilized structures. In this work, we propose an extension to QHA that fixes these problems. Our approach is based on four ingredients: (i) the calculation of the n-th order force constants using randomly displaced configurations and regularized regression, (ii) the calculation of temperature-dependent effective harmonic frequencies ω(V, T) within the self-consistent harmonic approximation (SCHA), (iii) Allen’s quasiparticle (QP) theory, which allows the calculation of the anharmonic entropy from the effective frequencies, and (iv) a simple Debye-like numerical model that enables the calculation of all other thermodynamic properties from the QP entropies. The proposed method is conceptually simple, with a computational complexity similar to QHA but requiring more supercell calculations. It allows incorporating anharmonic effects to any order. The predictions of the new method coincide with QHA in the low-temperature limit and eliminate the QHA blowout at high temperature, recovering the experimentally observed behavior of all thermodynamic properties tested. The performance of the new method is demonstrated by calculating the thermodynamic properties of geologically relevant minerals MgO and CaO. In addition, using cubic SrTiO3 as an example, we show that, unlike QHA, our method can also predict thermodynamic properties of dynamically stabilized phases. We expect this new method to be an important tool in geochemistry and materials discovery.

Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principle-driven methodologies to model complex chemical and materials processes. Over the past few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach, and outlook.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

Abstract The double carbonate BaCa(CO3)2 holds potential as host compound for carbon in the Earth’s crust and mantle. Here, we report the crystal structure determination of a high-pressure BaCa(CO3)2 phase characterized by single-crystal X-ray diffraction. This phase, named post-barytocalcite, was obtained at 5.7 GPa and can be described by a monoclinic Pm space group. The barytocalcite to post-baritocalcite phase transition involves a significant discontinuous 1.4% decrease of the unit-cell volume, and the increase of the coordination number of 1/4 and 1/2 of the Ba and Ca atoms, respectively. High-pressure powder X-ray diffraction measurements at room- and high-temperatures using synchrotron radiation and DFT calculations yield the thermal expansion of barytocalcite and, together with single-crystal data, the compressibility and anisotropy of both the low- and high-pressure phases. The calculated enthalpy differences between different BaCa(CO3)2 polymorphs confirm that barytocalcite is the thermodynamically stable phase at ambient conditions and that it undergoes the phase transition to the experimentally observed post-barytocalcite phase. The double carbonate is significantly less stable than a mixture of the CaCO3 and BaCO3 end-members above 10 GPa. The experimental observation of the high-pressure phase up to 15 GPa and 300 ºC suggests that the decomposition into its single carbonate components is kinetically hindered.

The seventh blind test of crystal structure prediction (CSP) methods substantially increased the level of complexity of the target compounds relative to the previous tests organized by the Cambridge Crystallographic Data Centre. In this work, the performance of density‐functional methods is assessed using numerical atomic orbitals and the exchange‐hole dipole moment dispersion correction (XDM) for the energy‐ranking phase of the seventh blind test. Overall, excellent performance was seen for the two rigid molecules (XXVII, XXVIII) and for the organic salt (XXXIII). However, for the agrochemical (XXXI) and pharmaceutical (XXXII) targets, the experimental polymorphs were ranked fairly high in energy amongst the provided candidate structures and inclusion of thermal free‐energy corrections from the lattice vibrations was found to be essential for compound XXXI. Based on these results, it is proposed that the importance of vibrational free‐energy corrections increases with the number of rotatable bonds. [ABSTRACT FROM AUTHOR]

The objective of crystal structure prediction (CSP) is to predict computationally the thermodynamically stable crystal structure of a compound from its stoichiometry or its molecular diagram. Crystal similarity indices measure the degree of similarity between two crystal structures and are essential in CSP because they are used to identify duplicates. Powder‐based indices, which are based on comparing X‐ray diffraction patterns, allow the use of experimental X‐ray powder diffraction data to inform the CSP search. Powder‐assisted CSP presents two unique difficulties: (i) the experimental and computational structures are not entirely comparable because the former is subject to thermal expansion from lattice vibrations, and (ii) experimental patterns present features (noise, background contribution, varying peak shapes etc.) that are not easily predictable computationally. This work presents a powder‐based similarity index (GPWDF) based on a modification of the index introduced by de Gelder, Wehrens & Hageman [J. Comput. Chem. (2001), 22, 273–289] using cross‐correlation functions that can be calculated analytically. Based on GPWDF, a variable‐cell similarity index (VC‐GPWDF) is also proposed that assigns a high similarity score to structures that differ only by a lattice deformation and which takes advantage of the analytical derivatives of GPWDF with respect to the lattice parameters. VC‐GPWDF can be used to identify similarity between two computational structures generated using different methods, between a computational and an experimental structure, and between two experimental structures measured under different conditions (e.g. different temperature and pressure). VC‐GPWDF can also be used to compare crystal structures with experimental patterns in combination with an automatic pre‐processing step. The proposed similarity indices are simple, efficient and fully automatic. They do not require indexing of the experimental pattern or a guess of the space group, they account for deformations caused by varying experimental conditions, they give meaningful results even when the experimental pattern is of very poor quality, and their computational cost does not increase with the flexibility of the molecular motif. [ABSTRACT FROM AUTHOR]

Measurement(s) bond separation energies Technology Type(s) ab initio quantum chemistry computational method Factor Type(s) existing or hypothetical model structure Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.16680787

Phase change materials (PCMs), which bear a strong relationship with pnictogens (group V or 15) and chalcogens (group VI or 16), are mostly chalcogens related to AIVBVI and A V2 B VI3 families. The exceptional properties and technological applications of PCMs have sparked interest in the nature of the unconventional chemical bonding present in the crystalline phases of PCMs, which has been reported as resonant, hypervalent, electron-rich multicenter, three-center-four-electron, and metavalent bonding along the last seventy years. This unconventional bond is also expected to occur at high pressure in most pnictogens, chalcogens, and AIVBVI and A V2 B VI3 compounds that are not PCMs at room pressure. These compounds are characterized at room pressure by a mixture of primary covalent ppσ-bonds and secondary bonds in which the lone electron pairs (LEPs) are involved. In this work, we provide evidence of the existence of an unconventional bonding (similar to that of PCMs) in the highpressure phases of pnictogens and chalcogens using theoretical simulations. We also unravel the mechanism of its formation and how it depends on the type of LEP present in secondary bonds. Moreover, we show that the unconventional bond of PCMs is the electron-deficient multicenter bond. This comprehensive understanding of chemical bonding in pnictogens and chalcogens, which can be extrapolated to advanced materials, such as PCMs, will play a crucial role in explaining the structure and properties of advanced materials as well as in enhancing their applications. [ABSTRACT FROM AUTHOR]

The quantum theory of atoms in molecules (QTAIM), developed by Bader and co-workers, is one of the most popular ways of extracting chemical insight from the results of quantum mechanical calculations. One of the basic tasks in QTAIM is to locate the critical points of the electron density and calculate various quantities (density, Laplacian, etc.) on them since these have been found to correlate with molecular properties of interest. If the electron density is given analytically, this process is relatively straightforward. However, locating the critical points is more challenging if the density is known only on a three-dimensional uniform grid. A density grid is common in periodic solids because it is the natural expression for the electron density in plane-wave calculations. In this article, we explore the reconstruction of the electron density from a grid and its use in critical point localization. The proposed reconstruction method employs polyharmonic spline interpolation combined with a smoothing function based on the promolecular density. The critical point search based on this reconstruction is accurate, trivially parallelizable, works for periodic and non-periodic systems, does not present directional lattice bias when the grid is non-orthogonal, and locates all critical points of the underlying electron density in all tests studied. The proposed method also provides an accurate reconstruction of the electron density over the space spanned by the grid, which may be useful in other contexts besides critical point localization. [ABSTRACT FROM AUTHOR]

Many crystal structure prediction protocols only concern themselves with the electronic energy of molecular crystals. However, vibrational contributions to the free energy (Fvib) can be significant in determining accurate stability rankings for crystal candidates. While force-field studies have been conducted to gauge the magnitude of these free-energy corrections, highly accurate results from quantum mechanical methods, such as density-functional theory (DFT), are desirable. Here, we introduce the PV17 set of 17 polymorphic pairs of organic molecular crystals, for which plane wave DFT is used to calculate the vibrational free energies and free-energy differences (ΔFvib) between each pair. Our DFT results confirm that the vibrational free-energy corrections are small, having a mean value of 1.0 kJ/mol and a maximum value of 2.3 kJ/mol for the PV17 set. Furthermore, we assess the accuracy of a series of lower-cost DFT, semi-empirical, and force-field models for computing ΔFvib that have been proposed in the literature. It is found that calculating Fvib using the Γ-point frequencies does not provide ΔFvib values of sufficiently high quality. In addition, ΔFvib values calculated using various approximate methods have mean absolute errors relative to our converged DFT results of equivalent or larger magnitude than the vibrational free-energy corrections themselves. Thus, we conclude that, in a crystal structure prediction protocol, it is preferable to forego the inclusion of vibrational free-energy corrections than to estimate them with any of the approximate methods considered here. [ABSTRACT FROM AUTHOR]

Atomic and molecular dispersion coefficients can now be calculated routinely using density-functional theory. In this work, we present the first determination of how electronic excitation affects molecular [C.sub.6] London dispersion coefficients from the exchange-hole dipole moment (XDM) dispersion model. Excited states are typically found to have larger dispersion coefficients than the corresponding ground states, due to their more diffuse electron densities. A particular focus is both intramolecular and intermolecular charge-transfer excitations, which have high absorbance intensities and are important in organic dyes, light-emitting diodes, and photovoltaics. In these classes of molecules, the increase in [C.sub.6] for the electron-accepting moiety is largely offset by a decrease in [C.sub.6] for the electron-donating moiety. As a result, the change in dispersion energy for a chromophore interacting with neighbouring molecules in the condensed phase is minimal. Key words: London dispersion, excited states, density-functional theory. Les coefficients de dispersion atomiques et moleculaires peuvent desormais etre calcules couramment au moyen de la theorie de la fonctionnelle de la densite. Dans le cadre des presents travaux, nous sommes les premiers a presenter une maniere de determiner comment l'excitation electronique influe sur les coefficients de dispersion moleculaire [C.sub.6] de London a partir du modele de dispersion base sur le moment dipolaire associe au trou d'echange (XDM). Les etats excites montrent habituellement de plus grands coefficients de dispersion que leurs etats fondamentaux correspondants en raison de leur densite electronique plus diffuse. Nous avons concentre notre attention sur les excitations de transfert de charge intramoleculaire et intermoleculaire dont l'intensite d'absorbance est elevee et qui representent un interet pour les domaines des colorants organiques, des diodes electroluminescentes et de la photovoltaïque. Dans ces classes de molecules, l'augmentation du coefficient [C.sub.6] du fragment accepteur d'electrons est largement compensee par la diminution de ce meme coefficient pour le fragment donneur d'electrons. Par consequent, la variation de l'energie de dispersion d'un chromophore interagissant avec les molecules voisines dans la phase condensee est minime. [Traduit par la Redaction] Mots-cles : dispersion de London, etats excites, theorie de la fonctionnelle de la densite., Introduction The study of electronic excitations is essential in many areas of chemistry. Molecular excitations play important roles in the design and fabrication of organic electronics (1,2) (sensors, light-emitting diodes, [...]

In the solid state, the title compound, C18H13N5O, adopts a conformation whereby the phenyl ring and methoxy–benzene-1,2-dicarbonitrile residue (r.m.s. deviation of the 12 non-H atoms = 0.041 Å) lie to opposite sides of the central triazolyl ring, forming dihedral angles of 79.30 (13) and 64.59 (10)°, respectively; the dihedral angle between the outer rings is 14.88 (9)°. This conformation is nearly 7 kcal mol−1 higher in energy than the energy-minimized structure which has a syn disposition of the outer rings, enabling intramolecular π–π interactions. In the crystal, methylene-C—H...N(triazolyl) and carbonitrile-N...π(benzene) interactions lead to supramolecular chains along the a axis. Supramolecular layers in the ab plane arise as the chains are connected by benzene-C—H...N(carbonitrile) interactions; layers stack with no directional interactions between them. The specified intermolecular contacts along with other, weaker contributions to the supramolecular stabilization are analysed in a Hirshfeld surface analysis.