The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements

Advances in solid-state NMR methodology and computational chemistry are applied to the (19)F NMR of solid octafluoronaphthalene. It is demonstrated experimentally, and confirmed by density functional theory (DFT) calculations, that the spectral resolution in the magic-angle spinning spectrum is limited by the anisotropy of the bulk magnetic susceptibility (ABMS). This leads to the unusual observation that the resolution improves as the sample is diluted. DFT calculations provide assignments of each of the peaks in the (19)F spectrum, but the predictions are close to the limits of accuracy and correlation information from 2-D NMR is invaluable in confirming the assignments. The effects of non-Gaussian lineshapes on the use of 2-D NMR for mapping correlations of spectral frequencies (e.g. due to the ABMS) are also discussed.

Carbonates are the major hosts of carbon on Earth's surface and their fate during subduction needs to be known to understand the deep carbon cycle. Magnesite (MgCO 3 ) is thought to be an important phase participating in deep Earth processes, but its phase stability is still a matter of debate for the conditions prevalent in the lowest part of the mantle and at the core mantle boundary. Here, we have studied the phase relations and stabilities of MgCO 3 at these P, T conditions, using Raman spectroscopy at high pressures (∼148 GPa) and after heating to high temperatures (∼3600 K) in laser-heated diamond anvil cell experiments. The experimental Raman experiments were supplemented by x-ray powder diffraction data, obtained at a pressure of 110 GPa. Density-functional-theory-based model calculations were used to compute Raman spectra for several MgCO 3 high-pressure polymorphs, thus allowing an unambiguous assignment of Raman modes. By combining the experimental observations with the density-functional-theory results, we constrain the phase stability field of MgCO 3 with respect to the high-pressure polymorph, MgCO 3 -II. We further confirm that Fe-free MgCO 3 -II is a tetracarbonate with monoclinic symmetry (space group C2/m), which is stable over the entire P, T range of the Earth's lowermost mantle geotherm.

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of Pb2SnO4 strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator-to-semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory-based calculations show that at ambient conditions the channels in the structure host the stereochemically-active Pb 6s 2 lone electron pairs. On compression the lone electron pairs form bonds between Pb2+ ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands.

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The stopping site of the muon in a muon-spin relaxation experiment is in general unknown. There are some techniques that can be used to guess the muon stopping site, but they often rely on approximations and are not generally applicable to all cases. In this work, we propose a purely theoretical method to predict muon stopping sites in crystalline materials from first principles. The method is based on a combination of ab initio calculations, random structure searching, and machine learning, and it has successfully predicted the Mu and Mu stopping sites of muonium in Si, diamond, and Ge, as well as the muonium stopping site in LiF, without any recourse to experimental results. The method makes use of Soprano, a Python library developed to aid ab initio computational crystallography, that was publicly released and contains all the software tools necessary to reproduce our analysis.

We report on the experimental characterization of the room-temperature organic ferroelectric croconic acid using neutron and X-ray diffraction, as well as optical and neutron spectroscopy. The crystallographic and spectroscopic neutron data provide detailed information on the location and underlying vibrational motions of the hydrogen atoms and allow the unambiguous identification of two distinct types of hydrogen-bonding motifs within the crystal structure. The experimental data are also used to assess a series of first-principles models of this material within the framework of density functional theory. The inclusion of non-local van der Waals interactions within the generalized-gradient approximation proves essential to provide a self-consistent picture of the observed optical response in the visible region of the electromagnetic spectrum, the crystal structure, and the main vibrational features associated with intermolecular hydrogen-bonding interactions in this single-component organic solid.

A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5-300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature.