Ab initio SCF CI study of the electronic spectra of nitroethylene

Configuration interaction studies of ground, n→π* and π→π* electronically excited states are reported for nitroethylene in its ground-state equilibrium geometry. In case of the n→π* transitions, the two symmetry combinations of oxygen lone pair orbitals are found to give almost the same transition energies of 4·42 eV and 4·48 eV. These are in good agreement with the experimental transition energy of 4·20 eV. The calculated transition energy to the lowest π→π* excited ¹ A′ state of 5·48 eV is also in good agreement with the reported experimental value of 5·12 eV. Numerous other singlet states as well as the triplet states have also been calculated.     

Ab initio study of the ground and excited states of hydrogen sulphide using SCF and CI calculations

Results from ab initio SCF and CI calculations on the ground state and low-lying valence and Rydberg states of H₂S are reported. A double ξ basis of contracted gaussian functions augmented by polarization and diffuse 3d, 4s and 4p functions is used for the calculations. The geometries of various excited states are studied by means of SCF calculations. The first observed band in the absorption spectrum is predicted to arise from the overlapping of transitions from the 2b₁ orbital to a Rydberg 4s and strongly bent valence upper state. The calculations support the assignment of other spectral features to transitions from the 2b₁ to components of the Rydberg 3d and 4p orbitals.     

Ab initio SCF and limited CI calculations of the d-d transitions in cobalt oxide

Ab initio calculations using an extended set of gaussian-type orbitals have been performed for the CoO₆ ^10- cluster in the Hartree-Fock approximation and with a limited configuration interaction. The results have been compared with those of crystal-field theory. CoO is found to be a very ionic system with localized d-electrons. The excited d-states may be described using a simple configuration interaction scheme, which is found to be almost equivalent to a crystal-field parameterized configuration interaction built on Hartree-Fock results. Localization and delocalization phenomena in openshell systems have been discussed.     

Ab initio study of electronic structure of strained

Micro-hardness and scratch adhesion testing are the most commonly used techniques for assessing the mechanical properties of thin films. Both of these testing methods utilize single-point contact and induce plastic deformation in the substrate and film. However, the influence of adhesion on the measured hardness has been seldom reported so far. In our experiments, diamond-like carbon (DLC) and silicon carbide (SiC) films deposited on silicon and nickel-based alloy substrates by pulsed laser ablation were indented and scratched by a Vickers micro-hardness tester and a diamond-cutter, respectively. It was found that the composite hardness decreased more rapidly for poor adhesion when increasing the indentation load. The result was explained by the elastic-plastic deformation mode of indentation and helped us to understand the physical meaning of one parameter commonly introduced in the models used to separate film hardness from the composite hardness. Received 30 June 1998     

Ab initio MRD-CI investigation of the electronic spectrum of glyoxal (CHO)2

Multireference configuration interaction (MRD-CI) calculations are used to compute the electronic spectrum of glyoxal (CHO)₂, a key species in atmospheric chemistry. The calculations place the first dipole-allowed transition 1¹A_u←X¹A_g at 2.80?eV (442.8?nm) with an oscillator strength of 0.0002 and the dipole forbidden 1¹B_g?←?X¹A_g transition at 4.22?eV (293.8?nm), in accordance with prior experiments. In addition, a much stronger transition (3¹B_u?←?X¹A_g) at 8.51?eV (145.7?nm) is predicted, which has not yet been reported in the literature. This transition corresponds to 1b_g?→?2a_u excitation and can be characterized as π(CO)?→?π?(CO) type in accordance with the computed relatively large oscillator strength f?=?0.38. The corresponding triplet states are also computed.     

Ab initio SCF LCAO band structure of poly(SN)

An ab initio SCF LCAO crystal orbital calculation has been performed for the one-dimensional poly(SN) chain. The band structure obtained contains two overlapping (a σ and a π) partially filled bands of a combined width of 6.9 eV.     

Ab initio study of the electronic and transport properties of waved graphene nanoribbons

We apply the nonequilibrium Green's function method based on density functional theory to investigate the electronic and transport properties of waved zigzag and armchair graphene nanoribbons. Our calculations show that out-of-plane mechanical deformations have a strong influence on the band structures and transport characteristics of graphene nanoribbons. The computed I-V curves demonstrate that the electrical conductance of graphene nanoribbons is significantly affected by deformations. The relationship between the conductance and the compression ratio is found to be sensitive to the type of the nanoribbon. The results of our study indicate the possibility of mechanical control of the electronic and transport properties of graphene nanoribbons.     

Ab initio investigation of the electronic structure of AgCl

The structural and electronic behavior of a prototype silver halide, namely AgCl, has been investigated by means of the full-potential linearized augmented plane wave method with the local density approximation. The aim of our study is to focus on the influence of the d metal orbitals through a detailed analysis of the electronic energies. The presence the d Ag orbitals leads to a strong hybridization between them and the p halogen ones, giving rise to specific electronic states organization and charge distribution. As expected from its rock-salt structure, AgCl is as an ionic compound characterized by a mixed p–d top valence band.     

Ab initio study of the electronic and atomic structure of the wolframite-type ZnWO4

Ab initio quantum chemistry calculations of the structural and electronic properties of monoclinic wolframite-type ZnWO₄ crystal have been performed within the periodic linear combination of atomic orbitals (LCAO) method using six different Hamiltonians, based on density functional theory (DFT) and hybrid Hartree-Fock-DFT theory. The obtained results for optimized structural parameters, band gap and partial density of states are compared with available experimental data, and the best agreement is observed for hybrid Hamiltonians. The calculations show that zinc tungstate is a wide band gap material, with the direct gap about 4.6 eV, whose valence band has largely O 2p character, whereas the bottom of conduction band is dominated by W 5d states.     

Ab initio study of structural,electronic and dynamical properties of MgAuSn

The structural and electronic properties of MgAuSn in the cubic AlLiSi structure have been studied, using density functional theory within the local density approximation. The calculated lattice constant for MgAuSn is found to be in good agreement with its experimental value. Our calculated electronic structure is also compared in detail with a recent tight-binding. A linear-response approach to density-functional theory is used to calculate the phonon spectrum and density of states for MgAuSn.     

Ab initio rovibrational spectrum of LiNC and LiCN

Rovibrational calculations are performed on an ab initio potential energy surface for lithium cyanide. Vibrational states localized about both the isocyanide structure and the metastable cyanide structure are found. Calculated fundamental frequencies are LiNC 126.6 cm^?1 (bend) and 754.3 cm^?1 (stretch); LiCN 165.8 cm^?1 (bend) and 688.8 cm^?1 (stretch). Many states are found in the region of the barrier to isomerization, some of which are delocalized (polytopic).     

Ab initio many-body treatment of the electronic structure of metals

We propose and apply a combination of an ab initio (band-structure) calculation with a many-body treatment including screening effects. We start from a linearized muffin-tin orbital (LMTO) calculation to determine the Bloch functions for the Hartree one-particle Hamiltonian, from which we calculate the static susceptibility and dielectric function within the standard random phase approximation (RPA). From the Bloch functions we obtain maximally localized Wannier functions, using a method proposed by Marzari and Vanderbilt. Within this Wannier basis all relevant one-particle and unscreened and screened Coulomb matrix elements are calculated. This yields a multi-band Hamiltonian in second quantization with ab initio parameters, for which screening has been taken into account within the simplest standard approximation. Then, established methods of many-body theory are used. We apply this concept to a simple metal, namely lithium (Li). Here the maximally localized Wannier functions turn out to be of the sp³-orbital kind. Furthermore, only the on-site contributions of the screened Coulomb matrix elements are relevant, and a generalized, four-band Hubbard model is justified. The screened on-site Coulomb matrix elements are considerably smaller than the band width because of which it is sufficient to calculate the selfenergy in weak-coupling approximation. We compare results obtained within the screened Hartree-Fock approximation (HFA) and within the second-order perturbation theory (SOPT) in the Coulomb matrix elements for Li and find that many-body effects are small but not negligible even for this simple metal.     

Ab initio calculation of the spin-orbit coupling constant from gaussian lobe SCF molecular wavefunctions

The spin-orbit coupling constants of BH⁺ and CH have been calculated using ab initio molecular SCF wavefunctions with the gaussian lobe function basis set. It is demonstrated that fair agreement with experimental values can be achieved even with a relatively small basis set, provided that no terms are neglected in calculating the matrix elements.     

Ab initio study on the electronic states and laser cooling of AlCl and AlBr

We investigate whether AlCl and AlBr are promising candidates for laser cooling. We report new ab initio calculations on the ground state X~1Σ~+ and two low-lying states(A~1Π and a~3Π) of AlCl and AlBr. The calculated spectroscopic constants show good agreement with available theoretical and experimental results. We also obtain the permanent dipole moments(PDMs) curve at multi-reference configuration interaction(MRCI) level of theory. The transition properties of A~1Π and a~3Π states are predicted, including the transition dipole moments(TDMs), Franck–Condon factors(FCFs), radiative times and radiative width. The calculated radiative lifetimes are of the order of a nanosecond, implying that they are sufficiently short for rapid laser cooling. Both AlCl and AlBr have highly diagonally distributed FCFs which are crucial requirement for molecular laser cooling. The results demonstrate the feasibility of laser cooling AlCl and AlBr, and we propose laser cooling schemes for AlCl and AlBr.     

Ab initio studies of the electronic properties of the silica surface

Self-consistent unrestricted Hartree-Fock calculations for the surface of crystalline silica (α-quartz) have been performed. We report here the one-electron energy levels and wavefunctions for several models of surface geometry, and a comparison is then made with experimentally determined densities of states.     

Ab initio study of the stability and electronic properties of wurtzite and zinc-blende BeS nanowires

In this work we study the structural stability and electronic properties of the Beryllium sulfide nanowires (NWs) in zinc-blende (ZB) and wurtzite (WZ) phases (with triangle and hexagonal cross sections), using first principle calculations within the plane-wave pseudopotential method. A phenomenological model is used to explain the role of dangling bonds in the stability of the NWs. In contrast to the bulk phase, the ZB-NWs with diameters less than 133.3 Å are found to be less favorable over the WZ-NWs, in which the surface dangling bonds (DBs) on the NW facets play an important role to stabilize the NWs. Furthermore, both ZB- and WZ-NWs are predicted to be semiconductor and the values of the band gaps are dependent on the surface DBs as well as the size and shape of the NWs. Finally, we obtain atom projected density of states (PDOSs) by calculating the localized density of states on the surface atoms, as well as on the core and edge atoms.