Two distinct points on the potential energy curve (PEC) of a pairwise interaction, the zero‐energy crossing point and the point where the stretching force constant vanishes, allow us to anticipate the range of possible distances between two atoms in diatomic, molecular moieties and crystalline systems. We show that these bond‐stability boundaries are unambiguously defined and correlate with topological descriptors of electron‐density‐based scalar fields, and can be calculated using generic PECs. Chemical databases and quantum‐mechanical calculations are used to analyze a full set of diatomic bonds of atoms from the s‐p main block. Emphasis is placed on the effect of substituents in C−C covalent bonds, concluding that distances shorter than 1.14 Å or longer than 2.0 Å are unlikely to be achieved, in agreement with ultra‐high‐pressure data and transition‐state distances, respectively. Presumed exceptions are used to place our model in the correct framework and to formulate a conjecture for chained interactions, which offers an explanation for the multimodal histogram of O−H distances reported for hundreds of chemical systems.

The chemical pressure approach offers a new paradigm for property control in functional materials. In this work, we disclose a correlation between the β → α pressure-induced phase transition in SnMoO₄ and the substitution process of Mo⁶⁺ by W⁶⁺ in SnMo_1-xW_xO₄ solid solutions (x = 0-1). Special attention is paid to discriminating the role of the lone pair Sn²⁺ cation from the structural distortive effect along the Mo/W substitution process, which is crucial to disentangle the driven force of the transition phase. Furthermore, the reverse α → β transition observed at high temperature in SnWO₄ is rationalized on the same basis as a negative pressure effect associated with a decreasing of W⁶⁺ percentage in the solid solution. This work opens a versatile chemical approach in which the types of interactions along the formation of solid solutions are clearly differentiated and can also be used to tune their properties, providing opportunities for the development of new materials.

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.

The valence shell electron pair repulsion (VSEPR) model is a demanding testbed for modern chemical bonding formalisms. The challenge consists in providing reliable quantum mechanical interpretations of how chemical concepts such as bonds, lone pairs, electronegativity, or hypervalence influence (or modulate) molecular geometries. Several schemes have been developed thus far to visualize and characterize these effects; however, to the best of our knowledge, no scheme has yet incorporated the analysis of the premises derived from the ligand close-packing (LCP) extension of the VSEPR model. Within the LCP framework, the activity of the lone pairs of the central atom and ligand–ligand repulsions constitute the two key features necessary to explain certain controversial molecular geometries that do not conform to the VSEPR rules. Considering the dynamical picture obtained when electron local forces at different nuclear configurations are evaluated from first-principles calculations, we investigate the chemical pressure distributions in a variety of molecular systems, namely, electron-deficient molecules (BeH₂, BH₃, BF₃), several AX₃ series (A: N, P, As; X: H, F, Cl), SO₂, ethylene, SF₄, ClF₃, XeF₂, and nonequilibrium configurations of water and ammonia. Our chemical pressure maps clearly reveal space regions that are totally consistent with the molecular and electronic geometries predicted by VSEPR and provide a quantitative correlation between the lone pair activity of the central atom and electronegativity of ligands, which are in agreement with the LCP model. Moreover, the analysis of the kinetic and potential energy contributions to the chemical pressure allows us to provide simple explanations on the connection between ligand electronegativity and electrophilic/nucleophilic character of the molecules, with interesting implications in their potential reactivity. NH₃, NF₃, SO₂, BF₃, and the inversion barrier of AX₃ molecules are selected to illustrate our findings.

Two distinct points on the potential energy curve (PEC) of a pairwise interaction, the zero‐energy crossing point and the point where the stretching force constant vanishes, allow us to anticipate the range of possible distances between two atoms in diatomic, molecular moieties and crystalline systems. We show that these bond‐stability boundaries are unambiguously defined and correlate with topological descriptors of electron‐density‐based scalar fields, and can be calculated using generic PECs. Chemical databases and quantum‐mechanical calculations are used to analyze a full set of diatomic bonds of atoms from the s‐p main block. Emphasis is placed on the effect of substituents in C−C covalent bonds, concluding that distances shorter than 1.14 Å or longer than 2.0 Å are unlikely to be achieved, in agreement with ultra‐high‐pressure data and transition‐state distances, respectively. Presumed exceptions are used to place our model in the correct framework and to formulate a conjecture for chained interactions, which offers an explanation for the multimodal histogram of O−H distances reported for hundreds of chemical systems.