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Finite-temperature properties such as thermodynamic state functions can be obtained for a range of crystalline materials by combining density functional theory (DFT) with lattice-dynamics approaches. Despite the usefulness of such first-principles predictions, their results must be carefully checked for accuracy, especially in cases where the DFT description of the material itself is nontrivial. Here, we investigate a prototypical layered semiconductor, namely, germanium selenide (GeSe) by dispersion-corrected DET, lattice-dynamics computations, and a thermodynamic framework that relies on the quasiharmonic approximation (QHA). We study phonon band structures, their evolution under pressure, and finite-temperature thermodynamic state functions. Besides the layered orthorhombic structure, this analysis includes the high-temperature cubic (rocksalt-type) polymorph of GeSe, which is shown to exhibit imaginary vibrational modes but emerging dynamic stability under increasing external pressure. The effect of these imaginary vibrational modes on the QHA and hence on computed thermochemical properties is critically evaluated. First-principles thermodynamics correctly predict a high-temperature transition from the orthorhombic to the cubic structure, albeit the transition temperature is severely underestimated. This simple compound allows us to address important methodological questions regarding the QHA treatment of crystalline solids. [ABSTRACT FROM AUTHOR]

Transition-metal borides show not only promising physical properties but also a rich variety of crystal structures. In this context, quantum-chemical tools can shed light on important facets of the chemistry within such intermetallic borides. Using density-functional theory (DFT), we analyze in detail two phases of significant structural-chemical importance: the recently synthesized Ti1+xOs2-xRuB2 and the isotypical Ti1+xOs3-xB2. Starting from the observation of different Ti/Os occupations in X-ray crystal structure analysis, we assess suitable computational models and rationalize how the interplay of Ti-Ti, Ti-Os, and Os-Os bonds drives the site preferences. Then, we move on to a systematic investigation of the metal-boron bonds which embed the characteristic, trigonal-planar B4 units within their metallic surroundings. Remarkably, the different Ti-B bonds in Ti1+xOs2-xRuB2 (and also in its ternary derivative) are of vastly different strength, and the strength of these bonds does not correlate with their length. The tools presented in this work are based on simple and insightful chemical arguments together with DFT, and may subsequently be transferred to other intermetallic phases-transition-metal borides and beyond. [ABSTRACT FROM AUTHOR]

Hydrogen bonding is among the most important interactionsin molecularcrystals, and examples are abundant. As a consequence of such interactions,many molecules crystallize in complex but intriguing structures, incontrast to the relatively simple packing principles of metallic orionic solids. In this work, we present a computational approach basedon plane-wave density-functional theory (DFT) and supercell techniques,aiming to understand and quantify hydrogen-bonded networks in thesolid state and in two-, one-, and zero-dimensional fragments derivedfrom the molecular crystal. With such methodology at hand, we investigateguanidine, a fitting example of a molecular crystal and an importantcompound for inorganic and organic chemistry alike. On the basis ofour computations, we discuss the initially proposed layered structureof guanidine and identify both stabilizing and destabilizing cooperativeinteractions in the three crystalline dimensions. [ABSTRACT FROM AUTHOR]

Organic molecules crystallize in manifold structures.The lastfew decades have seen the rise of high-resolution X-ray diffractiontechniques that make the structures of even the most complex crystalseasily accessible. Still, an intrinsic challenge lies in assigninghydrogen atoms’ positions from X-ray experiments alone. Quantumchemistry plays a fruitful, complementary role here, and so ab initiooptimization techniques for organic crystals are on the rise as well.In this context, we review and evaluate a popular ab initio strategybased on plane-wave density-functional computations, namely, selectivelyrelaxing H positions in an otherwise fixed cell. Our data show thatsuch-optimized C–H, N–H, O–H, and B–Hbond lengths coincide well with results from neutron diffractiontheexperimental technique that sets the “gold standard”for H positions in molecular crystals but which is far less easilyavailable. We have thus justified the use of a quantum-chemical aidewith a broad variety of possible applications. [ABSTRACT FROM AUTHOR]