Discrete Variational Xα Research Articles

Ab initio study of the 57Fe quadrupole splitting in the heme models of α- and β-subunits in tetrameric deoxyhemoglobin

Ab initio Xα discrete variation method was used for calculation of quadrupole splitting for the rough heme models in α- and β-subunits of tetrameric deoxyhemoglobin accounting small stereochemical variations. The differences of theoretical values of quadrupole splitting for these heme models were obtained.

First principles study of the alloying effect on chemical bonding characteristics of helium in La–Ni–M tritides

The alloying effect on the electronic structure of La–Ni–M tritides is investigated using the first principles discrete variational Xα(DV-Xα) method. The calculated results show that the covalent interaction between atoms will play a much more important role in studying the alloying effect on chemical bonding characteristics in La–Ni–M tritides than ionic interaction. It is also found that in La–Ni–M tritides helium forms stronger covalent bonds with the weaker hydride forming elements than La. By analyzing the relation between the binding energy difference and bond order, our study indicates that after some alloying elements substituting for Ni locating in 3g site in tritides, the helium retention capability becomes stronger, changes as the following sequence: Al > Cr > Mn > Fe > Co > Ni, and is also very distinct for Cu although the chemical bonding between Cu atom and Ni atom is degraded drastically.

범밀도함수법을 이용하여 계산한 IIIB족 원소가 도핑된 ZnO의 전자상태

The electronic states of ZnO doped with Al, Ga and In, which belong to III family elements in periodic table, were calculated using the density functional theory. In this study, the calculation was performed by two Programs; the discrete variational Xa (DV-Xa) method, which is a sort of molecular orbital full potential method; Vienna Ab-initio Simulation Package (VASP), which is a sort of pseudo potential method. The fundamental mixed orbital structure in each energy level near the Fermi level was investigated with simple model using DV-Xa. The optimized crystal structures calculated by VASP were compared to the measured structures. The density of state and the energy levels of dopant elements were shown and discussed in association with properties.

An X-ray Spectral Study of the Electronic Structure of Uranyl Fluoride UO2F2

The fine structure of the X-ray photoelectron spectrum of low-energy electrons and O 4,5(U) X-ray emission spectrum of uranyl fluoride UO2F2 (at binding energies from 0 to 40 eV) was identified on the basis of the electronic structure calculations for the [(UO2)F6]4− cluster with D 6h point symmetry, simulating the nearest surrounding of uranium in UO2F2, by the relativistic X α discrete variation method. It was shown that the U5f electrons can directly participate in chemical bonding, and U6p electrons, in formation of not only inner valence but also outer valence molecular orbitals. The sequence of the inner valence molecular orbitals in the binding energy region from 12 to 40 eV was established, which is essential for development of a procedure for determining the uranium-ligand bond length in the axial direction and in the equatorial plane of the uranyl compounds, based on the characteristics of their X-ray photoelectron spectra.

Critical thickness for ferroelectricity of BaTiO3 by first-principles calculations

The critical thickness for ferroelectricity of a BaTiO3 film was determined by the first-principles discrete variational-Xα molecular orbital method and population analysis. Under the assumption that most ferroelectric perovskite oxides are predominantly ionic, a series of model clusters were developed for a BaTiO3 particle consisting of a Ba8Ti7O6 cluster constructed on the basis of the crystal structure of BaTiO3 phase and point charges surrounding the Ba8Ti7O6 cluster. The size of the model cluster was exactly defined by the size of a three-dimensional point-charge array. By comparing the dependence of the net charge of Ba, Ti, and O ions and of the overlap population between Ti3d and O2p orbitals on particle thickness in tetragonal and cubic BaTiO3 particles, the critical thickness for ferroelectricity was calculated to be about 12 nm for a BaTiO3 particle with a basal area of 20×20nm.

Nature of the Chemical Bond in Uranium Dioxide UO2

Fine structure of the X-ray photoelectron and conversion spectra of low-energy (0–40 eV) electrons of uranium dioxide UO2 was analyzed based on the electronic structure calculations for the UO 8 12− cluster with O h symmetry, simulating the nearest surrounding of uranium in UO2, by the relativistic X α discrete variation method. It was predicted theoretically and validated experimentally that, in UO2, the U5f electrons (∼1 U5f electron) can directly participate in chemical bonding: ∼2 U5f electrons weakly contributing to chemical bonding are localized at −1.9 eV; ∼1 U5f electron participating in chemical bonding is delocalized in the range of outer valence molecular orbital energies from −4 to −9 eV; and unfilled U5f states are localized mostly at low (from 0 to 5 eV above zero) energies. It was shown experimentally that the U6p electrons actively participate in formation of not only inner valence but also outer valence (0.6 U6p electron) molecular orbitals. The density of the U6p states in UO2 was estimated experimentally. The composition and sequence of the inner valence molecular orbitals at energies within 13–40 eV were also elucidated.

パラジウム<110>対称傾角粒界の原子構造と電子状態

In this study, the energy and atomic structure of the symmetric tilt boundaries in palladium were calculated using a molecular dynamics (MD) method and the electronic structures of hydrogen in the bulk and at the grain boundaries were calculated using discrete-variational Xα (DV-Xα) molecular orbital cluster calculaion by solving Hartree-Fock-Slater equation. The result of MD simulation revealed that the energy of the symmetric tilt boundary of palladium depended on the misorientation angle and that there were large energy cusps at the misorientation angles which correspond to the {111}Σ3 and {113}Σ11 symmetric tilt boundaries. The atomic structure of all symmetric tilt boundaries could consist of the combination of the {331} Σ19, {111}Σ3 and {113}Σ11 structural units and {110}Σ1 and {001}Σ1 single crystal units. The result of DV-Xα molecular orbital cluster calculation showed that the interstitial hydrogen atoms induced the palladium-hydrogen chemical bond which had a different energy level from the palladium-palladium bond. The component of the palladium-hydrogen bond at the grain boundaries was similar to those in the bulk palladium. It is clarified that electronic structure near the grain boundary is different from that in the perfect crystal.

Near edge x-ray absorption fine structure studies of local structure of dihexyldisulfide multilayer

The multiple-scattering cluster (MSC) method is employed to calculate the S 1s near edge x-ray absorption fine structure (NEXAFS) of single dihexyldisulfide molecule and dihexyldisulfide multilayer. The local structure model of the dihexyldisulfide multilayer is proposed. The MSC calculation shows that the neighboring dihexyldisulfide molecules are parallelly arranged in an ordered square cross section with side of 0.47 nm. The discrete variational Xα method is employed to calculate the electronic structure of the single molecule and multilayer, which confirms the MSC results, and reveals the physical origin of all the features in the S 1s NEXAFS spectrum. The analysis of the interaction between the dihexyldisulfide molecules shows that the local structure of the multilayer has the property of self-assemblage.

Studies of Effects of Adsorption of Silicon or Germanium on the Electronic States of (100) GaAs Surfaces

The electronic states of both bare Ga-terminated and As-terminated (100)-(1×1) GaAs surfaces and the same surfaces with adsorbed Si or Ge were studied by molecular orbital calculations using small cluster models and the discrete variational Xα method. The most stable adsorption sites for Si and Ge are bridge sites on the (100)-GaAs surface, regardless of whether they are Ga- or As-terminated. When Si or Ge atoms are adsorbed, the density of the surface states in the forbidden band of the Ga or As surface layers markedly decreases. This suggests that both the Ga-terminated and As-terminated surfaces have a clear band gap when either Si or Ge are adsorbed on the surfaces. These adsorbates have states in the lower half of the band gap or in the valence band.

In situ real-time spectroscopic ellipsometry study of HfO2 thin films grown by using the pulsed-source metal-organic chemical-vapor deposition

The HfO2 thin-film growth process is investigated by using in situ real-time spectroscopic ellipsometry (SE) technique combined with the first-principles molecular-orbital (MO) calculations of the electronic states. The HfO2 films are grown on the silicon substrate by using the pulsed-source metal-organic chemical-vapor deposition method. Particular attention is paid to the formation of an interfacial layer at the early stage of the growth process by monitoring energy-dependent dielectric constants of the film. The energy dependence of the electronic polarizabilities and dielectric constants is calculated for the amorphous HfO2, SiO2, and HfSiO4 films based on the electronic states and density of states obtained using the discrete-variational Xα MO method with the unit cluster model. The measured SE spectra show that the average dielectric constants of the film vary gradually from those for SiO2 to those for HfO2 when the number of deposition cycles increases. By comparing the varied dielectric constants during the film growth with the calculated results, we find that the HfO2 film growth process can be divided into two stages with different growth mechanisms: SiO2 and HfxSiyOz layers are grown at the first stage, which are regarded as the interfacial layers, and the HfO2 layer formation becomes predominant at the second stage.

First principles study of effect of lattice misfit on the bonding strength of Ni/Ni3Al interface

By using a discrete variational Xα (DV-Xα) method, the electronic structures and bonding strengths of Ni/Ni3Al (or γ/γ′) interface with different lattice misfits (δ) were calculated in the framework of the nonrelativistic first-principles theory. In order to describe the effect of δ on the interfacial binding strength and the structural stability of coherent γ/γ′ interface, we calculated the interfacial binding covalent bond density (CBD) and the local environmental total bond overlap population (LTBOP). Very obvious effects of lattice misfits on the electronic structures of coherent γ/γ′ interface were found. On one hand, less than −0.6% negative lattice misfit can increase the binding strength of the γ/γ' interface. On the other hand, the local environmental total bonding strength of the γ/γ' interface decreases with increasing magnitude of δ. Therefore, the magnitude and sign of lattice misfit must be carefully controlled to balance the high-temperature creep strength of Ni-base single crystal superalloy and the structural stability of the γ/γ' interface when one designs new alloys.

DV-X α calculation of electron energy-loss near edge-structures of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4TCNQ)

The first principle molecular orbital (MO) calculations, focusing on the carbon K-edge fine structures, were performed for 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F 4TCNQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Electron energy-loss spectra (EELS) of F 4TCNQ were measured and analyzed through the calculation. In the discrete variational-Xα (DV-Xα) method, site-excited transition state configurations were assumed to take account of core-hole effects and, consequently, were examined in three parts: (1) partial density of states (PDOS) of unoccupied 2p orbitals; (2) inner 1s orbitals (energy-recalibrated PDOS); and (3) photo-absorption cross-section (PACS) considering the transition matrix. In F 4TCNQ, the inner 1s energy level of the quinoid-ring carbon combined to fluorine shifts deeper in energy, resulting in a high-energy shift of the π ∗ peaks in the calculation. The calculation also shows the fluorination effect on the quinoid-ring carbons appearing in the higher σ ∗ energy region. In the low energy region, the calculations accurately reproduced the core-excited spectra in both F 4TCNQ and TCNQ and succeeded in assigning each component appearing in the spectra.

A molecular orbital study on the chemical interaction across the indomethacin/silica interparticle boundary due to mechanical stressing

A discrete variational-Xα (DV-Xα) method was applied to elucidate the formation of C–O–Si bridging bonds, produced when a silanol group of SiO 2 comes close enough to react with a carboxyl group of indomethacin (IM). A decrease in the coordination number (CN) of oxygen atoms per silicon atom at the SiO 2 surface increased the net charge (NC) of the silanolic hydrogen atom and decreased the bond overlap population (BOP) between the silanolic oxygen and hydrogen atoms. These changes favored the formation and stabilization of C–O–Si bridging bonds with simultaneous dehydration. The computational results agreed well with our previous experimental study of a ground indomethacin–SiO 2 mixture to obtain a better solid dispersion of the drug.

Effect of Chemisorbed Oxide Layers Modified by Nitrogen and Sulfur Atoms on the Catalytic Properties of Platinum Metal Electrodes: A Quantum-Chemical Approach

Electrocatalytic properties of platinum metal electrodes and their dependence on nitrogen- and sulfur-containing additives to electrolyte are studied by an Xαdiscrete variation method. It is shown that rhodium catalysts are the most promising, while iridium is unsuitable for the synthesis of percompounds.

Atomic and Electronic Structure of ‹110› Symmetric Tilt Boundaries in Palladium

In the present study, the energy and atomic structure of the ‹110› symmetric tilt boundaries in palladium were evaluated using the molecular dynamics (MD) simulation and the electronic structures of hydrogen in the bulk and on the grain boundaries of palladium were calculated using the discrete-variational Xα (DV-Xα) method. The MD simulation revealed that the energy of the ‹110› symmetric tilt boundary of palladium depended on the misorientation angle and that there were large energy cusps at the misorientation angles which corresponded to the (111)Σ3 and (113)Σ11 symmetric tilt boundaries. The atomic structure of all ‹110› symmetric tilt boundaries could consist of the combination of the (331)Σ19, (111)Σ3 and (113)Σ11 structural units and (110)Σ1 and (001)Σ1 single crystal units. The DV-Xα calculation showed that the interstitial hydrogen atoms in palladium induced the Pd-H chemical bond which had a different energy level than the Pd-Pd bond. The energy level and component of the Pd-H bonding on the grain boundaries in palladium were similar to those in the bulk palladium.

Electronic Structure and Chemical Bonding of Zr Substitution of Ti Site on Pb(Zr1 −xTix)O3 Using DV-Xα Method

Electronic structures were investigated on Pb(Zr1 −xTix)O3 (PZT) using the discrete variational Xα cluster method for obtaining the information of dependence on the amount and position of the substituted Zr atoms. Also, we investigated the effect of them on fatigue. The net charge and the overlap population were discussed together with the energy gap, which is related to fatigue. As a result, the net charge was independent on the amount and position of the substituted Zr atoms, and the overlap population wasn't. Furthermore, from the calculated energy gap, we conclude that Zr-rich PZT is less susceptible to fatigue than Ti-rich PZT.

Oxygen and Sulfur Adsorption Effects on Electronic States of GaAs(100) Surfaces Studied with Discrete Variational Xα Method

The electronic states of Ga-terminated and As-terminated GaAs(100)-(1×1) surfaces and those surfaces with adsorbed O and S are studied by molecular orbital calculation using small cluster models and the discrete variational Xα method. Without adsorption of any foreign atoms, the Ga-terminated surface has 4p-related surface states in the middle of the forbidden band. While, the As-terminated surface has 4p-related surface states in the lower half of the forbidden band. The adsorption sites of chalcogen atoms on Ga- and As-terminated surfaces are determined ab initio by using density functional theory (Gaussian 98). With adsorbed chalcogen atoms, the density of the surface states in the forbidden band of the surface Ga layer markedly decreases, and that of the surface As layer increases. The adsorbates have states in the lower half of the band gap (Ga-terminated surface) or in the valence band (As-terminated surface). The results suggest that a chalcogen adsorbed Ga-terminated and bare As-terminated surface have low surface densities in the upper half of the band gap.

Electronic structure of acceptor-donor complexes in silicon

The electronic structure of trimer acceptor-donor complexes in silicon Si clusters is studied using the ab initio discrete variational-Xα molecular-orbital (MO) method. The trimer complexes In2D (D=phosphorus P, arsenic As, antimony Sb, or bismuth Bi) consist of two indium In acceptor elements and a centered donor element D from the group V elements. Calculations are performed under the assumption that the three atoms are arranged in the nearest neighbor substitutional trimer configuration. Results indicate that the trimer complexes act as shallower acceptors having smaller ionization activation energies than In acceptor. The potential of In2D as an acceptor in Si is then discussed and In2D is proposed as a promising acceptor for the formation of channels and source/drains in ultralarge scaled integration.

Optical properties and electronic structure of yttrium oxysulfide

Reflectivity spectra of single crystals of Y 2 O 2 S have been measured at 8 and 300 K in the energy range up to 30 eV using synchrotron radiation as a light source. The dielectric constants, absorption coefficient, and effective number of electrons per molecule are derived through a Kramers-Kronig analysis. The x-ray photoelectron spectroscopy has also been performed, in order to get information on the valence band and the outermost core band. With use of the discrete variational Xa (DV-Xa) method, the electronic structure of a model cluster consisting of an yttrium ion surrounded by three sulfur ions and four oxygen ions has been calculated. The calculation indicates that the exciton absorption peak observed at 6.33 eV is associated with the transition from the S 3p state to the Y 4d state. The band gap energy is estimated to be 6.77 eV. The electronic structure of Y 2 O 2 S is discussed on the basis of the present experimental and theoretical results, and is compared with that of Y 2 O 3 .

The theoretical study of O2 adsorption on NiTi (100) and (110) surfaces

The discrete variational Xa method (DV-Xα) within the framework of density-functional theory was applied to study O2 molecule adsorption on NiTi (100) and (110) surfaces. The bond order and charge distribution between Ti and O atoms for two possible O2 molecule adsorption ways on NiTi (100) and (110) surfaces were calculated. It is found that the adsorption way for O−O bond perpendicular to NiTi surface is preferred to that for O−O bond parallel to NiTi surface, and O2 molecule only interacted with one nearest surface titanium atom during the adsorption process. Mulliken population and the partial density of state analysis show that the interaction between Ti and O atoms is mainly donated by O 2p and Ti 4s electrons on NiTi(110) surface, O 2p and Ti 4s, 4p electrons on NiTi(100) surface, respectively. The total density of state analysis shows that NiTi(100) surface is more favorable for O2 molecule adsorption.