The crystal orbital bond index (COBI) is a new and intuitive method for quantifying covalent bonding in solid-state materials. COBI is based on the bond index by Wiberg and Mayer and extends their ideas to the case of translationally invariant objects, that is, crystalline matter. COBI’s qualitative interpretation resembles the well-established crystal orbital overlap population and crystal orbital Hamilton population methods but should be more familiar to chemists since it directly relates to the classical bond order. In contrast to the aforementioned descriptors, COBI also allows for examining multicenter interactions within a local-orbital framework. As an additional bonding indicator, we refer to the Ewald sum for electrostatic lattice potentials, thereby enabling the calculation of electrostatic lattice energies as well as site potentials from quantum-mechanical charges as directly derived from the wave function, not from the density.

The solid form landscape of 5-HT2a antagonist 3-(4-(benzo[d]isoxazole-3-yl)piperazin-1-yl)-2,2-dimethylpropanoic acid hydrochloride (B5HCl) proved difficult to establish. Many crystalline materials were produced by solid form screening, but few forms readily grew high quality crystals to afford a clear picture or understanding of the solid form landscape. Careful control of crystallization conditions, a range of experimental methods, computational modeling of solvate structures, and crystal structure prediction were required to see potential arrangements of the salt in its crystal forms. Structural diversity in the solid form landscape of B5HCl was apparent in the layer structures for the anhydrate polymorphs (Forms I and II), dihydrate and a family of solvates with alcohols. The alcohol solvates, which provided a distinct packing from the neat forms and the dihydrate, form layers with conserved hydrogen bonding between B5HCl and the solvent, as well as stacking of the aromatic rings. The ability of the alcohol hydrocarbon moieties to efficiently pack between the layers accounted for the difficulty in growing some solvate crystals and the inability of other solvates to crystallize altogether. Through a combination of experiment and computation, the crystallization problems, form stability, and desolvation pathways of B5HCl have been rationalized at a molecular level.

Chemical bonding in main‐group IV chalcogenides is an intensely discussed topic, easily understandable because of their remarkable physical properties that predestine these solid‐state materials for their widespread use in, for instance, thermoelectrics and phase‐change memory applications. The atomistic origin of their unusual property portfolio remains somewhat unclear, however, even though different and sometimes conflicting chemical‐bonding concepts have been introduced in the recent years. Here, it is proposed that projecting phononic force‐constant tensors for pairs of atoms along differing directions and ranges provide a suitable and quantitative descriptor of the bonding nature for these materials. In combination with orbital‐based quantitative measures of covalency such as crystal orbital Hamilton populations (COHP), it is concluded that the well‐established many‐center and even n‐center bonding is an appropriate picture of the underlying quantum‐chemical bonding mechanism, supporting the recent proposal of hyperbonded phase‐change materials.

Layered phase‐change materials in the Ge−Sb−Te system are widely used in data storage and are the subject of intense research to understand the quantum‐chemical origin of their unique properties. To uncover the nature of the underlying periodic wavefunction, we have studied the interacting atomic orbitals including their phases by means of crystal orbital bond index and fragment crystal orbital analysis. In full accord with findings based on projected force constants, we demonstrate the role of multicenter bonding along straight atomic connectivities. While the resulting multicenter bonding resembles three‐center‐four‐electron bonding in molecules, its solid‐state manifestation leads to distinct long‐range consequences, thus serving to contextualize the material properties usually termed “metavalent”. Eventually we suggest multicenter bonding to be the origin of their astonishing bond‐breaking and phase‐change behavior, as well as the too small “van‐der‐Waals” gaps between individual layers.

Future technologies are in need of solid-state materials showing the desired chemical and physical properties, and designing such materials requires a proper understanding of their electronic structures.