Trapped states and transitions between them in double-well pseudopotentials

We analyze a model of a double-well pseudopotential (DWPP), based in the 1D Gross-Pitaevskii equation with a spatially modulated self-attractive nonlinearity. In the limit case when the DWPP structure reduces to the local nonlinearity coefficient represented by a set of two delta-functions, analytical solutions are obtained for symmetric, antisymmetric and asymmetric states. In this case, the transition from symmetric to asymmetric states, i.e., a spontaneous-symmetry-breaking (SSB) bifurcation, is subcritical. Numerical analysis demonstrates that the symmetric states are stable up to the SSB point, while emerging asymmetric states (together with all antisymmetric solutions) are unstable in the delta-function model. In a general model, which features a finite width of the nonlinear-potential wells, the asymmetric states quickly become stable, simultaneously with the switch of the bifurcation into the supercritical type. Antisymmetric solutions may also enjoy stabilization in the finite-width DWPP structure, demonstrating a bistability involving the asymmetric states. The symmetric states require a finite norm for their existence. A full diagram for the existence and stability of the trapped states is produced for the general model.     

A comparison between ab initio calculated and measured Raman spectrum of triclinic albite (NaAlSi3O8)

Albite is one of the most common minerals in the Earth's crust, and its polymorphs can be found in rocks with different cooling histories. The characteristic spectrum of vibration of the albite mineral reflects its structural Si/Al ordering. In this study, we report on the comparison between the Raman spectra measured on a natural and fully ordered (as deduced on the basis of single‐crystal X‐ray diffraction data) ‘low albite’, NaAlSi₃O₈, and those calculated at the hybrid Hartree–Fock/density functional theory level by employing the WC1LYP Hamiltonian, which has proven to give excellent agreement between calculated and experimentally measured vibrational wavenumbers in silicate minerals. All the 39 expected A_g modes are identified in the Raman spectra, and their wavenumbers and intensities, in different scattering configurations, correspond well to the calculated ones. The average absolute discrepancy is ~3.4 cm⁻¹, being the maximum discrepancy |Δv|_max ~ 10.3 cm⁻¹. The very good quality of the WC1LYP results allows for reliable assignments of the Raman features to specific patterns of atomic vibrational motion. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical and energy-loss spectra of ZnS from ab initio molecular dynamics simulation: Temperature effect

First-principles molecular dynamics simulation has been performed so as to investigate the optical and energy-loss spectra of ZnS. Features such as dielectric function, density of states, reflectivity, refractive index and energy-loss function have been studied. Furthermore, the influence of temperature on the optical properties of interest is reported and discussed.     

Prediction models for the Abraham hydrogen bond donor strength: comparison of semi‐empirical,ab initio,and DFT methods

Hydrogen bonding has a great impact on the partitioning of organic compounds in biological and environmental systems as well as on the shape and functionality of macromolecules. Electronic characteristics of single molecules, localized at the H‐bond (HB) donor site, are able to estimate the donor strength in terms of the Abraham parameter A. The quantum chemically calculated properties encode electrostatic, polarizability, and charge‐transfer contributions to hydrogen bonding. A recently introduced respective approach is extended to amides with more than one H atom per donor site, and adapted to the semi‐empirical AM1 scheme. For 451 organic compounds covering acidic ? CH, ? NH? , and ? OH groups, the squared correlation coefficient is 0.95 for the Hartree–Fock and density functional theory (B3LYP) level of calculation, and 0.84 with AM1. The discussion includes separate analyses for weak, moderate, and strong HB donors, a comparison with the performance of increment methods, and opportunities for consensus modeling through the combined use of increment and quantum chemical methods. Copyright © 2011 John Wiley & Sons, Ltd.     

A comparison between shake-up and UV charge-transfer transitions

The He(II) spectra of the unsubstituted metallocenes {M(η-C₅H₅)₂}, M  V, Cr, Mn, Fe, Co, Ni and Ru, and of {Mn(η-C₅H₄Me)₂} are reported; both He(I) and He(II) spectra of some decamethylmetallocenes {M(η-C₅Me₅)₂}, where M  Mg, V, Cr, Mn, Fe, Co and Ni, are also given. Intensity changes between the He(I) and He(II) spectra are shown to provide a reliable guide to band assignment. A ligand field treatment of the decamethylmanganocene cation, including limited configuration interaction, gives values for Δ₂, B and C; these values are also in good agreement with the photoelectron spectra of {M(η-C₅Me₅)₂} where M  V, Cr and Fe. Overlap between the ligand and metal “d” band structures prevents complete assignment in the cases of Co and Ni.     

An ab initio study of 5d noble metal nitrides: OsN2, IrN2, PtN2 and AuN2

The crystal structure, phonon stability, elasticity and electronic properties of four noble metal nitrides (PtN₂, IrN₂, OsN₂ and AuN₂) with three structural types (pyrite, marcasite and CoSb₂ structure) were studied by first principles calculations. In agreement with experiments and previous theoretical predictions, it is found that the most stable structure for OsN₂ is marcasite, for PtN₂ is pyrite, and for IrN₂ is the CoSb₂ structure. It is found that these three compounds are thermodynamically metastable with respect to solid N₂ and the metal at zero pressure. The structures are mechanically and dynamically stable. The lowest energy structure of AuN₂ is the CoSb₂ structure. The formation energy of AuN₂ is found to be very high compared to the other three nitrides studied here. This underlies the experimental difficulty in the synthesis of this compound. OsN₂ is found to be metallic, while IrN₂ and PtN₂ are both semiconductors.     

Vibrational properties and phase transitions in II-VI materials: lattice dynamics, ab initio studies and inelastic neutron scattering measurements

Inelastic neutron scattering measurements were carried out to determine the phonon density of states of ZnSe and interpreted with lattice dynamical computations (ab initio as well as a potential model). Calculations are also reported for other II-VI compounds, ZnTe and ZnS. Vibrational (phonon spectra and Grüneisen parameters), and thermal (negative thermal expansion and non-Debye specific heat) properties have been calculated and found to be in good agreement with available experimental data. This model has been further employed to study the pressure-induced solid-solid phase transitions exhibited by these compounds and the results have been compared with experimental data. Total energy calculations for zincblende and SC16 phases of ZnSe were carried out employing the pseudopotential approach under the local density approximation (LDA) as well as the generalized gradient approximation (GGA). The density functional perturbation theory is applied to study the vibrational properties of the zincblende and SC16 phases of ZnSe. An investigation of the pressure dependence of the phonon frequencies shows that the existence of the (experimentally undetected) SC16 phase as a thermodynamically stable high pressure phase is impeded due to dynamical instabilities. A detailed investigation of the polarization of phonons of different energies for the various phases of these compounds indicates that in the case of the zincblende phase the low energy modes are librational, while in the rocksalt phase the low energy modes are bending modes. Further, in ZnTe the low energy bending modes display a larger amplitude of bending than that in ZnSe and ZnS.     

Nanomechanical properties and phase transitions in a double-walled (5,5)@(10,10) carbon nanotube: ab initio calculations

The structure and elastic properties of (5,5) and (10,10) nanotubes, as well as barriers for relative rotation of the walls and their relative sliding along the axis in a double-walled (5,5)@(10,10) carbon nanotube, are calculated using the density functional method. The results of these calculations are the basis for estimating the following physical quantities: ultimate shear strengths and diffusion coefficients for relative sliding along the axis and rotation of the walls, as well as frequencies of relative rotational and translational oscillations of the walls. The commensurability-incommensurability phase transition is analyzed. The length of the incommensurability defect is estimated on the basis of ab initio calculations. It is proposed that a double-walled carbon nanotube be used as a plain bearing. The possibility of experimental verification of the results is discussed.     

A comparison between transitions induced by random and periodic fluctuations

It is shown that a certain class of nonlinear systems possesses a unique and stable stationary state when subjected to periodic dichotomous modulations of an external parameter. This result enables us to define a probability density for the system and to characterize its shape and support. We compare this probability density with the one obtained in the case that the external parameter fluctuates randomly like a Markovian dichotomous noise and discuss various fluctuation-induced transition phenomena. The effects of these two types of fluctuations are quite dissimilar: the random fluctuations give rise to a richer behavior. The results are applied to the Freedericksz transition in nematic liquid crystals.Fellow of the University of Texas at Austin.     

Excitons in carbon nanotubes: an ab initio symmetry-based approach

The optical absorption spectrum of the carbon (4,2) nanotube is computed using an ab initio many-body approach which takes into account excitonic effects. We develop a new method involving a local basis set which is symmetric with respect to the screw-symmetry of the tube. Such a method has the advantages of scaling faster than plane-wave methods and allowing for a precise determination of the symmetry character of the single-particle states, two-particle excitations, and selection rules. The binding energy of the lowest, optically active states is approximately 0.8 eV. The corresponding exciton wave functions are delocalized along the circumference of the tube and localized in the direction of the tube axis.     

The keto-enol equilibrium in substituted acetaldehydes: focal-point analysis and ab initio limit

High-level ab initio electronic structure calculations up to the CCSD(T) theory level, including extrapolations to the complete basis set (CBS) limit, resulted in high precision energetics of the tautomeric equilibrium in 2-substituted acetaldehydes (XH₂C-CHO). The CCSD(T)/CBS relative energies of the tautomers were estimated using CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z calculations with MP2/aug-cc-pVTZ geometries. The relative enol (XHC?=?CHOH) stabilities (ΔE _{e,CCSD(T)/CBS}) were found to be 5.98?±?0.17, ?1.67?±?0.82, 7.64?±?0.21, 8.39?±?0.31, 2.82?±?0.52, 10.27?±?0.39, 9.12?±?0.18, 5.47?±?0.53, 7.50?±?0.43, 10.12?±?0.51, 8.49?±?0.33, and 6.19?±?0.18?kcal?mol^?1 for X?=?BeH, BH₂, CH₃, Cl, CN, F, H, NC, NH₂, OCH₃, OH, and SH, respectively. Inconsistencies between the results of complex/composite energy computations methods Gn/CBS (G2, G3, CBS-4M, and CBS-QB3) and high-level ab initio methods (CCSD(T)/CBS and MP2/CBS) were found. DFT/aug-cc-pVTZ results with B3LYP, PBE0 (PBE1PBE), TPSS, and BMK density functionals were close to the CCSD(T)/CBS levels (MAD?=?1.04?kcal?mol^?1).     

Non-random Be-to-Zn substitution in ZnBeSe alloys: Raman scattering and ab initio calculations

We show how the 1-bond ↦ 2-mode percolation behaviour, as observed in the transverse optical (TO) Raman spectra of several A_1-xB_xC semiconductor (SC) alloys, can be used to detect and to estimate a possible deviation from randomness in the A↔B substitution. As a case study we focus on the Zn_1-xBe_xSe system, which shows an uniquely well-resolved percolation-type TO fine structure in the spectral range of Be–Se vibrations, and discuss differences in the Be–Se Raman signals from bulk crystals and epitaxial layers. In the epilayers, the longer (shorter) Be–Se bonds from the BeSe-like (ZnSe-like) region appear to be slightly but systematically over- (sub-) represented with respect to the random case. This indicates a trend towards local ordering in analogy with the known case of InGaP₂. The discussion is supported by ab initio insight into the phonon/bond length properties of a prototype ZnBeSe supercell at x ~ 0.5 in their dependence on the amount of CuPt ordering (the most frequent type of spontaneous ordering). Besides, the ab initio calculations reveal some singularity in the lattice dynamics/relaxation around the intermediate value of the order parameter η ~ 0.5, coinciding with the so far experimentally achievable limit of CuPt spontaneous ordering.     

Interfaces between Al2O3 and beta-Ni(1-x)Al(x) alloys: complex structures and Ab initio thermodynamics

A methodology to study the structural stability of binary alloy/Al2O3 interfaces is developed by expanding the conventional ab initio thermodynamics to include the dependence on alloy composition. Results on beta-Ni(1-x)Al(x)/Al2O3 interfaces predict the existence of two types of stable interfaces. The stable interface at equilibrium with Al-rich or strictly 1:1 alloy contains an Al2-terminated Al2O3 surface and continues with NiAl layers, and the interface at equilibrium with Ni-rich alloy has Al accumulation but continues with a Ni-rich and then NiAl layers. Works of separation for the two interfaces are close to each other.     

Multi-centre bonding in the carboranes: An ab initio and semi-empirical molecular orbital study

The electronic structures of the closo-carboranes 1,5-C₂B₃H₅, 1,2-C₂B₄H₆, 1,6-C₂B₄H₆ and 2,4-C₂B₅H₇ are discussed using the results of ab initio calculations in both minimal and extended bases of contracted gaussian type functions fitted to Slater type orbitals. A comparison is made with the results from semi-empirical INDO calculations, and the bonding in 1,7-C₂B₆H₈, 1,6-C₂B₇H₉, 5,9-C₂B₇H₁₃, 1,6-C₂B₈H₁₀ and the 1,2-, 1,7- and 1,12-isomers of C₂B₁₀H₁₂ studied using the latter method.

The delocalized molecular orbitals from both the ab initio and semi-empirical calculations are transformed to localized orbitals to provide an analysis of the multi-centre bonding in these electron-deficient molecules, which requires no prior assumptions about the nature of the bonding.     

Sequence dependent electron transport in wet DNA: ab initio and molecular dynamics studies

We combine molecular dynamics simulations and density functional theory to analyze the electrical structure and transmission probability in four different DNA sequences under physiological conditions. The conductance in these sequences is primarily controlled by interstrand and intrastrand coupling between low-energy guanine orbitals. Insertion of adenine-thymine base pairs between the guanine-cytosine rich domains acts as a tunneling barrier. Our theory explains recent length dependent conductance data for individual DNA molecules in water.