Interacting Quantum Atoms Method for Crystalline Solids.

An implementation of the Interacting Quantum Atoms method for crystals is presented. It provides a real space energy decomposition of the energy of crystals in which all energy components are physically meaningful. The new package ChemInt enables one to compute intra-atomic and inter-atomic energies, as well as electron population measures used for quantitative description of chemical bonds in crystals. The implementation is tested and applied to characteristic molecular and crystalline systems with different types of bonding.

Predominant information quality scheme for the essential amino acids: an information-theoretical analysis.

In this work we undertake a pioneer information-theoretical analysis of 18 selected amino acids extracted from a natural protein, bacteriorhodopsin (1C3W). The conformational structures of each amino acid are analyzed by use of various quantum chemistry methodologies at high levels of theory: HF, M062X and CISD(Full). The Shannon entropy, Fisher information and disequilibrium are determined to grasp the spatial spreading features of delocalizability, order and uniformity of the optimized structures. These three entropic measures uniquely characterize all amino acids through a predominant information-theoretic quality scheme (PIQS), which gathers all chemical families by means of three major spreading features: delocalization, narrowness and uniformity. This scheme recognizes four major chemical families: aliphatic (delocalized), aromatic (delocalized), electro-attractive (narrowed) and tiny (uniform). All chemical families recognized by the existing energy-based classifications are embraced by this entropic scheme. Finally, novel chemical patterns are shown in the information planes associated with the PIQS entropic measures.

ThPt2: A New Representative of Close Packed Tetragonal Structures.

ThPt2 crystallizes with unique type of structure (space group I4/mmm, a = 4.1565(1) Å, c = 14.3663(7) Å, Pearson symbol tI12), which belongs to the group of the close packed tetragonal structures. An analysis of the chemical bonding by the electron localizability approach reveals the formation of two-dimensional layered platinum anionic substructures interlinked by strongly polar bonds to Th. Measurements of magnetic susceptibility, electrical resistivity, and specific heat show ThPt2 to be diamagnetic with metallic type of electrical conductivity in good agreement with the calculated electronic structure (N(EF) = 0.9 states eV(-1) f.u.(-1)).

TM7TM'6B8 (TM = Ta, Nb; TM' = Ru, Rh, Ir): new compounds with [B6] ring polyanions.

The ternary boron compounds TM(7)TM'(6)B(8) (TM = Ta, Nb; TM' = Ru, Rh, Ir) were prepared by high-temperature thermal treatment of mixtures of the elements. An analysis of the chemical bonding by the electron density/electron localizability approach reveals formation of covalently bonded polyanions [B(6)] and [TM'(6)B(2)]. The cationic part of the structure contains separated TM cations. In agreement with the chemical bonding analysis and band structure calculations, all TM(7)TM'(6)B(8) compounds are metallic Pauli-paramagnets (TM' = Ru, Rh) or diamagnets (TM' = Ir).

Domain-averaged Fermi-hole analysis for solids.

The domain-averaged Fermi hole (DAFH) orbitals provide highly visual representation of bonding in terms of orbital-like functions with attributed occupation numbers. It was successfully applied on many molecular systems including those with non-trivial bonding patterns. This article reports for the first time the extension of the DAFH analysis to the realm of extended periodic systems. Simple analytical model of DAFH orbital for single-band solids is introduced which allows to rationalize typical features that DAFH orbitals for extended systems may possess. In particular, a connection between Wannier and DAFH orbitals has been analyzed. The analysis of DAFH orbitals on the basis of DFT calculations is applied to hydrogen lattices of different dimensions as well as to the solids diamond, graphite, Na, Cu and NaCl. In case of hydrogen lattices, remarkable similarity is found between the DAFH orbitals evaluated with both the analytical approach and DFT. In case of the selected ionic and covalent solids the DAFH orbitals deliver bonding descriptions, which are compatible with classical orbital interpretation. For metals the DAFH analysis shows essential multicenter nature of bonding.

Electron localization and delocalization indices for solids.

The electron localization and delocalization indices obtained by the integration of exchange-correlation part of pair density over chemically meaningful regions of space, e.g., QTAIM atoms are valuable tools for the bonding analysis in molecular systems. However, among periodic systems only few simplest models were analyzed with this approach until now. This contribution reports implementation and evaluation of the localization and delocalization indices on the basis of solid state DFT calculations. A comparison with the results of simple analytical model of Ponec was made. In addition, a small set of compounds with ionic (NaCl), covalent (diamond, graphite), and metallic (Na, Cu) bonding interactions was characterized using this method. Typical features of different types of bonding were discussed using the delocalization indices.

Electron localizability indicators ELI-D and ELIA for highly correlated wavefunctions of homonuclear dimers. I. Li2, Be2, B2, and C2.

Electron localizability indicators based on the parallel-spin electron pair density (ELI-D) and the antiparallel-spin electron pair density (ELIA) are studied for the correlated ground-state wavefunctions of Li(2), Be(2), B(2), and C(2) diatomic molecules. Different basis sets and reference spaces are used for the multireference configuration interaction method following the complete active space calculations to investigate the local effect of electron correlation on the extent of electron localizability in position space determined by the two functionals. The results are complemented by calculations of effective bond order, vibrational frequency, and Laplacian of the electron density at the bond midpoint. It turns out that for Li(2), B(2), and C(2) the reliable topology of ELI-D is obtained only at the correlated level of theory.

Electron localizability indicators ELI-D and ELIA for highly correlated wavefunctions of homonuclear dimers. II. N2, O2, F2, and Ne2.

Electron localizability indicators based on the electron pair density ELI-D and ELIA Electron localizability indicators ELI-D and ELIA based on the electron pair density are studied for the correlated ground-state wavefunctions of N(2), O(2), F(2), and Ne(2) diatomics. Different basis sets and reference spaces are used for the multireference configuration interaction method following the complete active space calculations to investigate the local effect of electron correlation on the extent of electron localizability in position space determined by the two indicators. The results are complemented by calculations of effective bond order, vibrational frequency, and Laplacian of the electron density at the bond midpoint. It turns out that for O(2) and F(2), the reliable topology of ELI-D is obtained only at the correlated level of theory.

Electron localizability indicators ELI and ELIA: the case of highly correlated wavefunctions for the argon atom.

Electron localizability indicators based on the same-spin electron pair density and the opposite-spin electron pair density are studied for correlated wavefunctions of the argon atom. Different basis sets and reference spaces are used for the multireference configuration interaction method following the complete active space calculations aiming at the understanding of the effect of local electron correlation when approaching the exact wavefunction. The populations of the three atomic shells of Ar atom in real space are calculated for each case.

Direct space decomposition of ELI-D: interplay of charge density and pair-volume function for different bonding situations.

The topological features, i.e., gradients and curvatures of the same-spin electron pair restricted electron localizability indicator (ELI-D) in position space are analyzed in terms of those of the electron density and the pair-volume function. The analysis of the topology of these constituent functions and their interplay on ELI-D attractor formation for a number of molecules representing chemically different bonding situations allows distinguishing between different chemical bonding scenarios on a quantum mechanical basis without the recourse to orbitals. The occurrence of the Laplacian of the electron density in the expression for the Laplacian of ELI-D allows us to establish a physical link between electron localizability and electron pairing as displayed by ELI-D and the role of Laplacian of the density in this context.

Insight into the informational-structure behavior of the Diels-Alder reaction of cyclopentadiene and maleic anhydride.

The course of the Diels-Alder reactions of cyclopentadiene and maleic anhydride were studied. Two reaction paths were modelled: endo- and exo-selective paths. All structures within the transient region were characterized and analyzed by means of geometrical descriptors, physicochemical parameters and information-theoretical measures in order to observe the linkage between chemical behavior and the carriage of information. We have shown that the information-theoretical characterization of the chemical course of the reaction is in complete agreement with its phenomenological behavior in passing from reactants to products. In addition, we were able to detect the main differences between the two reaction mechanisms. This type of informational analysis serves to provide tools to help understand the chemical reactivity of the two simplest Diels-Alder reactions, which permits the establishment of a connection between the quantum changes that molecular systems exert along reaction coordinates and standard physicochemical phenomenology. In the present study, we have shown that every reaction stage has a family of subsequent structures that are characterized not solely by their phenomenological behavior but also by informational properties of their electronic density distribution (localizability, order, uniformity). Moreover, we were able to describe the main differences between endo-adduct and exo-adduct pathways. With the advent of new experimental techniques, it is in principle possible to observe the structural changes in the transient regions of chemical reactions. Indeed, through this work we have provided the theoretical concepts needed to unveil the concurrent processes associated with chemical reactions.

Bonding indicators from electron pair density functionals.

The bonding analysis of a chemical system is usually based on some descriptors. Distinct approaches are used to generate the bonding descriptors, whereby the usefulness of a particular approach is emphasized by the desire to yield a description consistent with the examined effects. Thus, whereas the bond path from the electron density gradient field yields the connectivity of the atomic fragments, the orbital picture can easily rationalize the rotational rigidity of a double bond. On the other hand none of the former is able to describe the volume demand of the bonds, which can be accessed by descriptors originating from approaches using space partitioning. As a conceivable way to describe the bonding situation a class of functionals based on the electron pair density integrals in both, direct and momentum space, is proposed. The localizability indicators defined by those functionals are examined on several molecules.

Charge decomposition analysis of the electron localizability indicator: a bridge between the orbital and direct space representation of the chemical bond.

The novel functional electron localizability indicator is a useful tool for investigating chemical bonding in molecules and solids. In contrast to the traditional electron localization function (ELF), the electron localizability indicator is shown to be exactly decomposable into partial orbital contributions even though it displays at the single-determinantal level of theory the same topology as the ELF. This approach is generally valid for molecules and crystals at either the single-determinantal or the explicitly correlated level of theory. The advantages of the new approach are illustrated for the argon atom, homonuclear dimers N2 and F2, unsaturated hydrocarbons C2H4 and C6H6, and the transition-metal-containing molecules Sc(2)2+ and TiF4.