Aurora Costales

Non-nuclear maxima of the electron density on alkaline metals

The topological properties of the electron density of bcc alkaline metals (Li–Cs) is examined by means of Hartree–Fock and density functional calculations. Our best results indicate that lithium is the only alkaline metal showing non-nuclear maxima (NNM) at the room pressure and temperature experimental geometry. Sodium and potassium, but not rubidium and cesium, would also present NNM under an appropriate compression, even though the NNM in potassium would be residual at best and contain a negligible amount of electrons. Despite these differences, all five alkaline metals share a common tendency towards topological change that makes their behavior clearly distinct from what is typical in ionic, covalent and molecular crystals. When examined in a wide range of interatomic distances, the electron density of every metal follows a well defined topological sequence, with strong similarities across the five metals.

Structural and chemical stability of halide perovskites

Abstract The ab initio Perturbed Ion ( ai PI) quantum mechanical method is used to study the solid state reaction: AX + MX 2 → AMX 3 from a thermodynamical point of view. The reaction energy is first determined by means of static calculations (i.e. at null absolute temperature) on the ideal cubic structures of the components. The very difficult problem of determining the most stable crystal structure of a compound is then undertaken by examining the differences in energy among many structures reported for AX , MX 2 and AMX 3 compounds. Finally, the reaction energy is again examined in the light of those corrections, and the results are used to analyze the experimental data available on the synthesis of perovskites.

A multipolar approach to the interatomic covalent interaction energy

Interatomic exchange‐correlation energies correspond to the covalent energetic contributions to an interatomic interaction in real space theories of the chemical bond, but their widespread use is severely limited due to their computationally intensive character. In the same way as the multipolar (mp) expansion is customary used in biomolecular modeling to approximate the classical Coulomb interaction between two charge densities ρA(r) and ρB(r) , we examine in this work the mp approach to approximate the interatomic exchange‐correlation (xc) energies of the Interacting Quantum Atoms method. We show that the full xc mp series is quickly divergent for directly bonded atoms (1–2 pairs) albeit it works reasonably well most times for 1– n (n > 2) interactions. As with conventional perturbation theory, we show numerically that the xc series is asymptotically convergent and that, a truncated xc mp approximation retaining terms up to l1+l2=2 usually gives relatively accurate results, sometimes even for directly bonded atoms. Our findings are supported by extensive numerical analyses on a variety of systems that range from several standard hydrogen bonded dimers to typically covalent or aromatic molecules. The exact algebraic relationship between the monopole‐monopole xc mp term and the inter‐atomic bond order, as measured by the delocalization index of the quantum theory of atoms in molecules, is also established.

Theoretical study of the group-IV antisite acceptor defects in CdGeAs2

Native and impurity antisite point defects in CdGeAs2 are studied here using an embedded quantum cluster model based on density functional theory. The calculated geometric relaxations and spin densities of the antisite defects considered here show a clear and distinct difference in the nature of native (i.e. [GeAs]) and impurity (i.e. [CAs] and [SiAs]) antisite defects in CdGeAs2. For the native antisite acceptor, the hole appears to be delocalized in contrast to impurity antisites where the hole is mainly localized at the acceptor site.

Ionic properties of perovskites derived from topological analysis of their wave function

We present in this work a discussion on the quantitative bonding information that can be deduced from the topological analysis of the crystal wave function of 120 alkali halide perovskites. The formalism, recently presented, is a development of the theory of atoms in molecules of Bader into the domain of crystalline materials. We discuss the shape of the ions and show how the classical picture in terms of slightly deformed spheres is contained in the topological description. The nature of the chemical bond in these systems is depicted by means of graphical representation of the electron density and its Laplacian along the surfaces of the attraction basins. The ionicity of the crystals and the behaviour of the ionic radii are also briefly reviewed.

Topological properties of the electron density of solids and molecules. Recent developments in Oviedo

Some of the latest advances in the analysis of electron density are reviewed, including: (a) topological indices that provide a useful characterization of the global properties of the density; (b) specific results on some prototypical metal and low heteropolarity systems; and (c) calculation of the local curvature of the interatomic surface.

Beryllium Bonding in the Light of Modern Quantum Chemical Topology Tools

We apply several modern quantum chemical topology (QCT) tools to explore the chemical bonding in well established beryllium bonds. By using the interacting quantum atoms (IQA) approach together with electron distribution functions (EDF) and the natural adaptive orbitals (NAdOs) picture, we show that, in agreement with orbital-based analyses, the interaction in simple σ and π complexes formed by BeX2 (X = H, F, Cl) with water, ammonia, ethylene, and acetylene is dominated by electrostatic terms, albeit covalent contributions cannot be ignored. Our detailed analysis proves that several σ back-donation channels are relevant in these dimers, actually controlling the conformational preference in the π adducts. A number of one-electron beryllium bonds are also studied. Orbital invariant real space arguments clearly show that the role of covalency and charge transfer cannot be ignored.

Real-Space In Situ Bond Energies: Toward A Consistent Energetic Definition of Bond Strength

A rigorous definition of intrinsic bond strength based on the partitioning of a molecule into real-space fragments is presented. Using the domains provided by the quantum theory of atoms-in-molecules (QTAIM) together with the interacting quantum atoms (IQA) energetic decomposition, we show how an in situ bond strength, matching all the requirements of an intrinsic bond energy, can be defined between each pair of fragments. Total atomization or fragmentation energies are shown to be equal to the sum of these in situ bond energies (ISBEs) if the energies of the fragments are measured with respect to their in-the-molecule state. These energies usually lie above the ground state of the isolated fragments by quantities identified with the standard fragment relaxation or deformation energies, which are also provided by the protocol. Deformation energies bridge dissociation energies with ISBEs, and can be dissected by using well-known tools of real-space theories of chemical bonding. Similarly, ISBEs can be partitioned into ionic and covalent contributions, and this feature adds to the chemical appeal of the procedure. All the energetic quantities examined are observable and amenable, in principle, to experimental determination. Several systems, exemplifying the role of each energetic term presented herein, are used to show the power of the approach.

Emergent Scalar and Vector Fields in Quantum Chemical Topology

Several potentially useful scalar and vector fields that have been scarcely or even never used to date in Quantum Chemical Topology are defined, computed, and analyzed for a few small molecules. The fields include the Ehrenfest force derived from the second order density matrix, which does not show many of the spurious features encountered when it is computed from the electronic stress tensor, the exchange-correlation (xc) potential, the potential acting on one electron in a molecule, and the additive and effective energy densities. The basic features of the topology of some of these fields are also explored and discussed, paying attention to their possible future interest.

Effect of dielectric polarization on the properties of charged point defects in insulating crystals: the nitrogen vacancy in AlN

Large unit cell calculations of the properties of charged point defects in insulators largely neglect dielectric polarization of the crystal, because the periodically repeated cells are so small. Embedded quantum cluster calculations with shell-model crystals, representing a single defect in a large crystal, are able to represent the polarization more realistically. For such embedded quantum clusters, we evaluate the optical excitation energy for the nitrogen vacancy in charge state (+3): vN3+ in AlN. This is done with and without dielectric polarization of the embedding crystal. A discrepancy of a few per cent is found, when both ground and excited state orbitals are well-localized within the vacancy. We show that the discrepancy rises rapidly as the excited state becomes more diffuse. We conclude that an embedded cluster approach will be required for transitions that involve even somewhat diffuse states. The investigation is based on a new model for AlN that shows promise for quantitative accuracy.