On the atomistic origin of the polymorphism and the dielectric physical properties of beryllium oxide.

Polymorphic beryllium oxide has been theoretically investigated from first principles as regards orbital occupancies, chemical bonding, polarization, as well as dielectric properties. By means of Crystal-Orbital Bond Index (COBI) analysis, the important role of the 2p orbitals on beryllium has been elucidated, in particular in terms of the correlation between polarization and beryllium-atom displacement, including the impact of the latter on the covalency of the BeO bond. In addition, several structural possibilities for a Bex Mg1-x O solid solution have been investigated for a Be content between 6% and 22%; for those, dynamically stable structures have been found, displaying large polarization values, more covalent BeO bonds, and a tendency for tetrahedral Be coordination. The dynamically unstable structures, however, resemble rock-salt BeO in their local structural properties around the Be atom. High dielectric constants and band gaps indicating insulating behavior have been found for those.

Structural Transformation in LnHS (Ln = La, Nd, Gd, and Er) with Coordination Change between an S-Centered Octahedron and a Trigonal Prism.

Lanthanide hydride chalcogenides LnHSe and LnHTe (Ln = lanthanides) crystallize in two polymorphs, 2H and 1H structures (ZrBeSi-type and filled-WC-type structures, respectively), but the chemical origin of the structural selection is unknown. Here, we have expanded the LnHCh (Ch = O, Se, and Te) family to include LnHS (Ln = La, Nd, Gd, and Er) using high-pressure synthesis. LnHS adopts the 2H structure for large Ln (La, Nd, and Gd) and the 1H structure for small Er. We compared the two polymorphs using anion-centered polyhedra and found that in the compounds with large ionicity, the 2H structure with ChLn6 octahedra is stabilized over the 1H structure with ChLn6 trigonal prisms due to relatively small electrostatic repulsion, supported by analysis of Madelung energy, crystal orbital Hamilton population (COHP), and density of energy (DOE).

Embedded Localized Molecular-Orbital Representations for Periodic Wave Functions.

To more straightforwardly provide local chemical-bonding reasoning in crystalline matter, we introduce a new approach to generate a real-space analogue of periodic electronic structures using "exact" top-down frozen-density embedding calculations. Based on the obtained real-space electronic structure, we then construct localized molecular orbitals and evidence that our technique compares favorably against the commonly used Wannier method, both in terms of numerical efficiency and details of chemical bonding. The new method has been implemented into the LOBSTER software package and designed as a black-box approach, digesting any periodic electronic structure from the currently supported codes, i.e., VASP, Quantum ESPRESSO, and ABINIT.

Metal Hydride Vibrations: The Trans Effect of the Hydride.

trans-Dihydride complexes are important in many homogeneous catalytic processes. Here vibrational spectroscopy and density functional theory (DFT) methods are used for the first time to reveal that 4d and 5d metals transmit more effectively than the 3d metals influence of the ligand trans to the hydride and also couple the motions of the trans-hydrides more effectively. This property of the metal is linked to higher hydride reactivity. The IR and Raman spectra of trans-FeH2(dppm)2, trans-RuH2(PPh(OEt)2)4, and mer-IrH3(PiPr2CH2pyCH2PiPr2) provide M-H force constants and H-M-H interaction force constants that increase as FeII < RuII < IrIII. DFT methods are used to determine, for the first time, the effect of the metal ion (MnI, ReI, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) and ligands on the gap in wavenumbers between the symmetric νsymH-M-H and antisymmetric νasymH-M-H vibrational modes of hydrides that are mutually trans in d6 octahedral complexes. The magnitude of this gap reflects the degree of coupling of, or interaction between, these modes, and this is shown to be a distinctive property of the metal ion. The more polarizable 4d and 5d metal ions are found to have an average gap of 246 cm-1, while the 3d metals have only 90 cm-1. This has been verified experimentally for 3d, 4d, and 5d transition-metal trans-dihydrides, where both the IR and Raman spectra have been measured: trans-RuH2(PPh(OEt)2)4 (from the literature) and trans-FeH2(PPh2CH2PPh2)2 and mer-IrH3(PiPr2CH2pyCH2PiPr2) (this work). Because the 4d and 5d metal ions tend to be better catalysts for the hydrogenation of substrates with polar bonds, this gap may be a fundamental determinant of the kinetic hydricity of the catalyst. Finding the magnitude of this gap and a new estimate of the large hydride trans-effect (Δνt -235 cm-1) allows us to improve the simple equation reported previously, which allows a better estimate of νM-H.

Serenity: A subsystem quantum chemistry program.

We present the new quantum chemistry program Serenity. It implements a wide variety of functionalities with a focus on subsystem methodology. The modular code structure in combination with publicly available external tools and particular design concepts ensures extensibility and robustness with a focus on the needs of a subsystem program. Several important features of the program are exemplified with sample calculations with subsystem density-functional theory, potential reconstruction techniques, a projection-based embedding approach and combinations thereof with geometry optimization, semi-numerical frequency calculations and linear-response time-dependent density-functional theory. © 2018 Wiley Periodicals, Inc.

Accurate embedding through potential reconstruction: A comparison of different strategies.

Potential reconstruction is a powerful strategy for deriving accurate (sometimes called "exact") embedding potentials in the context of density-dependent embedding methods. It is particularly useful for partitioning covalent bonds in such fragment-based electronic-structure methods. While the general approach is well defined and easily explained, there are a number of choices to be made in practice, concerning, e.g., the specific reconstruction algorithm, the assignment of electrons to subsystems, or the initial guess potential. A general choice to be made is whether "exact" embedding potentials shall be derived for pre-defined target densities (top-down) or for approximate fragment densities that can be iteratively defined (bottom-up). Here, we compare the pros and cons of a variety of different variants of potential reconstruction, both in terms of conceptual issues and concerning their accuracy and efficiency. We also present several algorithmic improvements that can be crucial in critical cases of potential reconstruction, namely, we show (i) that a combination of basis-set and grid-based potential reconstruction schemes can lead to improved resulting densities, (ii) that similarly the combination of real-space and matrix-representation based potential reconstruction gives great advantages, and (iii) that the potential-matrix reconstruction by Zhang and Carter [J. Chem. Phys. 148, 034105 (2018)] can be made much more efficient by avoiding an explicit Hessian calculation. Additionally, we demonstrated (iv) that a double reconstruction, meaning a reconstruction of both the supersystem potential and the subsystem potential, may lead to beneficial error cancellation. We also address the question of consistent energetics derived from such reconstructed potentials.

Structural, kinetic, and theoretical studies on models of the zinc-containing phosphodiesterase active center: medium-dependent reaction mechanisms.

Dinuclear zinc(II) complexes [Zn(2)(bpmp)(mu-OH)](ClO(4))(2) (1) and [Zn(2)(bpmp)(H(2)O)(2)](ClO(4))(3) (2) (H-BPMP=2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol) have been synthesized, structurally characterized, and pH-driven changes in metal coordination observed. The transesterification reaction of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) in the presence of the two complexes was studied both in a water/DMSO (70:30) mixture and in DMSO. Complex 2 was not reactive whereas for 1 considerable rate enhancement of the spontaneous hydrolysis reaction was observed. A detailed mechanistic investigation by kinetic studies, spectroscopic measurements ((1)H, (31)P NMR spectroscopy), and ESI-MS analysis in conjunction with ab initio calculations was performed on 1. Based on these results, two medium-dependent mechanisms are presented and an unusual bridging phosphate intermediate is proposed for the process in DMSO.