Ab initio crystal orbital calculations on polyacetylene-polyethylene copolymers

The band structures of polyacetylene-polyethylene copolymers of various compositions have been calculated. The results have been compared to the band structures of pure polyacetylene and polyethylene, and possible implications for molecular electronic devices have been discussed.     

ZnSe弹性常数和相变的从头计算

利用平面波密度泛函理论研究了ZnSe从闪锌矿结构到盐石结构的相变.结果发现通过H相等得到的相变压力为16.8 GPa,与通过高压弹性常数值判断所得到的结果相符.     

Molecular properties of FeCO as derived from AB initio molecular orbital calculations

We have performed ab initio all-electron and pseudopotential MO calculations on FeCO with varying degrees of sophistication in the basis sets as well as in the calculational procedures. Basis sets with at least double zeta representation for valence orbitals as well as inclusion of correlation corrections in the MO calculations are the minimum requirements in order to arrive at a clear decision about the electronic ground state of FeCO. In contradiction to the suggestion given by Guenzburger et al. [23] we derive from total energy considerations and from comparing experimental and calculated C−O stretching force constants that the ground state is a spin-quintet (⁵∑⁻) and not a spin-triplet. Our calculated quadrupole splitting for MO equilibrium geometry of FeCO⁵∑⁻ deviates by about 25% from the value which was observed by Peden et al. [1] for FeCO in solid noble gas. However, the calculated electronic configuration of iron within FeCO⁵∑⁻ of about 3d^6.44s^0.8 is far from 3d⁶4s², which approximately is expected for FeCO when comparing isomer shifts of FeCO and Fe, both matrix-isolated in solid noble gas, i.e. −0.60 mm/s and −0.75 mm/s, respectively. Thus, at the moment the question remains open to what extent the molecular properties of FeCO are affected by the solid noble gas matrix.     

Ab initio path-integral molecular dynamics

A path-integral molecular dynamics technique for strongly interacting atoms using ab initio potentials derived from density functional theory is implemented. This allows the efficient inclusion of nuclear quantum dispersion in ab initio simulations at finite temperatures. We present an application to the quantum cluster H ₅ ⁺ .     

Ab initio calculations of supramolecular complexes of fullerene C60 with CdTe and CdS

This paper presents the results of ab initio quantum-chemical calculations of supramolecular complexes C₆₀CdHal, [C₆₀]₄CdHal, and [C₆₀]₆CdHal (Hal = S, Te), which simulate the defects forming in fullerite during the absorption or adsorption of cadmium telluride (sulfide). Calculations of the electronic structure of complexes with inclusion of their relaxation to the equilibrium state have been performed in terms of the density functional theory with the B3LYP hybrid functional. The obtained enthalpies of formation of complexes show that their formation leads to the energy gain of the order of 0.5–1.5 eV depending on the complex type. It has been shown that the formation of tetrahedral complexes [C₆₀]₄CdTe with the intercalated CdTe molecule is possible only with a considerable distortion of the tetrahedral void. The energy spectrum of low-lying excited electron states for the linear and octahedral complexes has been calculated. It has been found that a decrease in symmetry with the formation of complexes leads to the appearance of excited states of allowed singlet transitions in the electron spectrum, which are forbidden in optical spectra of initial components.     

Split-shell molecular orbital calculations on the diatomic alkali metal molecules

Absolute g-tensor calculations for planar hydrocarbon and for non-planar phenyl substituted hydrocarbon radicals are reported. The relevant interactions determining g are discussed. Calculations are performed on the basis of a second-order perturbation expansion. The electronic wavefunctions are obtained from a simplified version of Hoffmann's extended Hückel model (SEH), where all valence electrons are taken into account explicitly. For planar systems the observed linear dependence of g on the energy of the half filled π orbital is well reproduced. A qualitative analysis of this dependence, making restrictive assumptions about the σ electrons, was given earlier by Stone. The calculations for non-planar model systems reproduce the g-factor anomalies which are observed for highly twisted phenyl substituted hydrocarbon radicals. The results show the necessity of direct π-σ mixing and are consistent with recent investigations of the proton hyperfine couplings in such systems.     

INDO MO calculations for first row transition metal complexes

A general molecular orbital method using a modified INDO scheme is given for systems involving first row elements and first row transitional elements. The new hamiltonian matrix elements and the appropriate parameterizations are given together with results for a number of test cases. The splitting of the different spin terms arising from a given orbital configuration is satisfactorily accounted for and the energy differences involved agree well with spectroscopic data.     

Split-shell molecular orbital calculations on the heteronuclear alkali metal diatomics

One of the radicals formed in irradiated 5-iododeoxyuridine is shown unambiguously to be the α-iodo radical RCH₂-?(I)R′ formed by hydrogen atom addition to C₆. The ¹²⁷I hyperfine tensor components, A_x = + 90 G, A_y = (-) 50 G, A_z = (-)40 G are proposed as being characteristic of the coupling to be expected for α-iodo alkyl radicals. Hence a radical recently detected in irradiated iodoacetamide with a maximum hyperfine coupling of 250 G cannot have this structure. Possible alternative structures are discussed.

The way in which the E.S.R. spectra for the α-iodo radica in 5-iododeoxyuridine are modified by the quadrupole interaction from ¹²⁷I is described and hence an estimate of the quadrupole coupling is obtained.     

Quantum-mechanical molecular orbital calculations

The theory of generalized combined orbitals is outlined. This is a development of the theory put forward by Löwdin [3]. Certain spectral characteristics of benzene, diphenyl, and nitrobenzene are calculated and the results are compared with the theory.In conclusion the author wishes to thank N. A. Prilezhaevaya, V, I. Danilova and O. A. Ponomarev for many discussions and also L. G. Turovets for carrying out the calculations.     

Ab initio theory and calculations of X-ray spectra

There has been dramatic progress in recent years both in the calculation and interpretation of various x-ray spectroscopies. However, current theoretical calculations often use a number of simplified models to account for many-body effects, in lieu of first principles calculations. In an effort to overcome these limitations we describe in this article a number of recent advances in theory and in theoretical codes which offer the prospect of parameter free calculations that include the dominant many-body effects. These advances are based on ab initio calculations of the dielectric and vibrational response of a system. Calculations of the dielectric function over a broad spectrum yield system dependent self-energies and mean-free paths, as well as intrinsic losses due to multi-electron excitations. Calculations of the dynamical matrix yield vibrational damping in terms of multiple-scattering Debye–Waller factors. Our ab initio methods for determining these many-body effects have led to new, improved, and broadly applicable x-ray and electron spectroscopy codes. To cite this article: J.J. Rehr et al., C. R. Physique 10 (2009).     

Ab initio calculations to model anomalous fluorine behavior

Implanted fluorine is observed to behave unusually in silicon, manifesting apparent uphill diffusion and reducing diffusion and enhancing activation of boron. In order to investigate fluorine behavior, we calculate the energy of fluorine defect structures in the framework of density functional theory. In addition to identifying the ground-state configuration and diffusion migration barrier of a single fluorine atom in silicon, a set of energetically favorable fluorine defect structures were found (F(n)V(m)). The decoration of vacancies and dangling silicon bonds by fluorine suggests that fluorine accumulates in vacancy-rich regions, which explains the fluorine redistribution behavior reported experimentally.     

Ab initio calculations of the photoionization of diatomic molecules

A review is presented of the calculation of photoionization spectra, particularly in the spectral range where electron autoionization of diatomic molecules takes place. In addition to some interesting results obtained over years that compare favourably with experiment, the emphasis here is put on the relation between the methods developed for the calculation of observables associated with the continuum energy spectrum of the electrons and the Alchemy system of programs. This system of programs serves as a basis for initial and intermediate calculations. The examples presented show that diatomic molecules not only in gas phase but also oriented in space or physisorbed at surfaces may be studied readily.     

Ab initio fermi surface calculation for charge-density wave instability in transition metal oxide bronzes

The electronic structure of the charge density wave (CDW) bronze (PO2)(4)(WO3)(2m), m = 4, is determined using ab initio density functional theory. The calculation shows that the Fermi surface (FS) consists in the superposition of three one-dimensional FS's associated with three types of chains. The q dependence of the electronic response function calculated from the electronic structure quantitatively accounts for the anisotropy of the fluctuations probed by x-ray diffuse scattering. The results validate the hidden nesting mechanism proposed for the CDW transitions in this series of bronzes.     

Valency electron molecular orbital calculations

The self-consistent perturbation method used previously with the CNDO/2 approximations is generalized to obtain diamagnetic susceptibilities and nuclear shielding constants, as well as polarizabilities, for N₂, BF, CO and F₂ both with a minimal basis set and with a slightly extended set containing an extra 2p orbital. The importance of origin-dependent terms is pointed out. For quantities that depend on the first-order perturbed molecular orbitals, inclusion of the extra 2p orbitals produces results in better agreement with experiment.     

Ab initio calculation of the superconducting transition temperature

The superconducting transition temperature is calculated for a series of representative metals from a selfconsistent LMTO-bandstructure calculation. We carefully avoid any uncontrolled approximations apart from the use of a local exchange-correlation potential and the rigid-ion approximation for the electron-phonon interaction, Our results for V, Nb, Ta, Mo, W, Pd, Pt, Pb clearly indicate that these popular approximations are incapable of reproducing the observed transition temperatures.