Realization of prediction of materials properties by<Emphasis Type="Italic">ab initio</Emphasis> computer simulation

Ab initio treatment is becoming realistic to predict physical, chemical, and even mechanical properties of academically and industrially interesting materials. There is, however, some limitation in size and time of the system up to the order of several hundred atoms and ∼ 1 pico second, even if we use the fastest supercomputer efficiently. Therefore, it is very difficult to simulate realistic materials with grain boundaries and important reactions like diffusion in materials. To improve this situation, two ways have been invented. One way is to upgrade approximations to match the necessary levels according to inhomogeneous electron gas theory beyond the present day standard, i.e. local density approximation (LDA). The reason is simply that the system we are interested in is composed of many particles interacting with Coulomb forces governed by quantum mechanics. (Complete knowledge is available, and only what we should do is to make better approximations to explain the phenomena!). Another is to extract the necessary parameters from theab initio calculations on systems with limited number of atoms, and apply these results into cluster variation, direct, or any other sophisticated methods based on classical concepts such as statistical mechanics. In this paper, several typical examples recently worked out by our research group are introduced to indicate that these methodologies are actually possible to be successfully used to predict materials properties before experiments based on the present day state-of-art supercomputing systems. It includes scientific visualization of the results ofab initio molecular dynamics simulation on atom insertion process to C₆₀ and to carbon nanotube, tight-binding calculation of single electron conductance properties in nanotube to create nano-scale diode virtually by computer, which will be a base of future nanoscale electric device in nanometer size, Li + H reaction without Born-Oppenheimer approximation, structural phase transitions in perovskite materials under very high pressure in earth by direct method, and prediction of wavelength of emitted light from Na clusters with GW (G = Green function-vertex, W = screened Coulomb interaction) approximation.     

Environmentally dependent bond-order potentials: New developments and applications

The bond-order potentials (BOPs) idea employs the orthogonal two-centre tight-binding (TB) representation for the bond energy and the Harris-Foulkes approximation for the repulsive pairwise contribution. In the last ten years, although many efforts have been focused on theoretical calculations of the bond order expression, the BOPs still suffers from the uncertainty of how best to choose the semi-empirical TB parameters that enter the scheme. In this paper, we review recent developments to obtain the reliable and transferable BOPs which help to extend the accuracy and applicability to technologically important multi-component systems. Firstly, we have found that a simple pair potential is unsuitable for describing the environmental screening effects due to the s andp orbital overlap repulsion in transition metal alloys and therefore the inability to reproduce the negative Cauchy pressures exhibiting in strong covalent systems. By adding the environmental dependent repulsive term, the Cauchy pressure problem has been removed and we are now able to get the BOPs for studying dislocations, extended defects and mechanical properties of high-temperature intermetallic Ti-Al alloys. In particular, new results on the core structures and possible dissociation of different type of dislocations will be discussed. Secondly, we present the first derivation of explicit analytic expressions for environmental dependence of σ,π and δ bond integrals by inverting the non-orthogonal matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of bond integrals between Mo, Si and MoSi2 but also predicts the large discontinuities between first and second nearest neighbours forppσ, ppπ andddn even though absence of any discontinuity for theddσ bond integral. A new environmentally dependent BOPs has been developed forbcc-Mo indicating that the core structure of l/2 <111> screw dislocations is narrower than structures found in previous studies in agreement with recentab initio calculations. Finally, the new formalism will allow us to study the problem of medium range order found recently in amorphous materials with covalent bonding at large and realistic nanoscale. For the case ofa-C where the issue ofsp ² vssp ³ is very crucial for modelling amorphous structure we found that the σ and π bond integrals are not only transferable between graphite and diamond structures but they are also strongly anisotropic due to inter-plan bonding between graphite sheets.     

Canab initio simulation really predict properties of materials prior to experimental

In this paper we have presented all-electron full-potentialab initio simulation method with introduction of mixed-basis, and have cited several typical examples which indicate that it is possible to predict properties of materials prior to experimental. Based on theab initio calculation of the total energy, cluster variation, and direct methods function, it is possible to bridge the limited scheme of theab initio treatment to real complex materials. Furthermore, to overcome the limited computer power, we have developed parallel processing codes and tested their efficiencies as well.     

Canab initio simulation really predict properties of materials prior to actually carrying out the experiments?

In the present paper, all-electron full-potentialab initio simulation method with mixed-basis is introduced and several typical examples are indicated which successfully show the possibility of predicting properties of materials prior to actually carrying out the experiments. We have used theab initio calculation to extract energy parameters, and apply them to cluster variation and direct methods, which bridge the limited space ofab initio treatment to real complex materials. To overcome the limited computer power, we have also developed parallel processing codes and tested their efficiencies.     

Ab initio molecular orbital and molecular graphics studies of benzene adsorption in Na Y zeolite

The interaction of benzene with the framework of the Na⁺-exchanged form of Y-type zeolite is studied using a combination of ab initio molecular orbital calculations and molecular graphics. Using this combination a visual rationalization of the forces governing the adsorption process is developed. The observed adsorption sites, i.e., over the Sll Na⁺ and the center of the twelve ring, are graphically shown to represent optimal accommodation of steric and electrostatic forces between sorbate and sorbent. Calculated sorption energies for both sites show good agreement with experimental determinations.     

Structure determination at room temperature and phase transition studies aboveT c in ABi4Ti4O15 (A = Ba, Sr or Pb)

The room temperature structure of three compounds belonging to the Aurivillius family (n = 4), ABi₄Ti₄O₁₅ (A = Ba, Sr or Pb) has been analysed. BaBi₄Ti₄O₁₅ crystallizes in a tetragonalI4/mmm space group whereas SrBi₄Ti₄O₁₅ and PbBi₄Ti₄O₁₅ crystallize in the orthorhombic space group A2₁am. The starting model for the Sr and Pb analogues was derived fromab initio methods and refined using the Rietveld method. The cations Ba and Sr are disordered over the Bi sites while the Pb cation is found exclusively in the [Bi₂O₂]²⁺ layers. The TiO₆ octahedra are tilted with the Ti-O bonds forming zigzag chains along the “c” axis. The displacement of Bi atoms along the “a” axis might be responsible for ferroelectricity in these compounds. The high temperature X-ray data aboveT _c indicate no structural transition for A = Ba and Pb while A = Sr transforms to the tetragonal structure.     

Ab initio, crystal field and experimental spectroscopic studies of pure and Ni-doped KZnF3 crystals

Pure and Ni²⁺ doped KZnF₃ single crystals were studied using the combination of the DFT-based ab initio methods, crystal field theory and experimental spectroscopic techniques. The electronic, optical and elastic properties have been calculated and compared with available experimental data and good agreement was achieved. Elastic anisotropy of pure KZnF₃ was modeled; calculations of the sound velocity, Debye temperature, Grüneisen parameter and specific heat capacity were performed. Comparison of the calculated results for the pure and doped material, which is reported for the first time for the considered material, enabled to identify the changes in the optical and electronic properties, which are due to the introduced nickel impurity ions. In particular, it was shown that the lowest Ni 3d states appear in the host's band gap at about 1.0 eV above the valence band. The changes of the electron density distribution after doping were also shown. Microscopic analysis of the crystal field effects based on the performed ab initio calculations of the Ni²⁺ density of states at different external pressures enabled to estimate the constants of the electron–vibrational interaction, Huang-Rhys factor, Stokes shift and local bulk modulus around impurity ions. The crystal field calculations of the Ni²⁺ energy levels were performed to analyze and assign the experimental absorption spectrum. Such a combination of the ab initio and semi-empirical calculating techniques leads to a complementary picture of the physical properties of KZnF₃:Ni²⁺ and can be applied to other doped crystals.     

Charge Order in (TMTTF)2PF6 Investigated by Infrared Spectroscopy

To explore novel physical phenomena related to strong π-d interaction, we measured the resistivity (ρ) and magnetoresistance of the first organic ferrimagnetic π-d system, (EDT-TTFVO)₂FeBr₄ under high pressures up to 8 GPa, where EDT-TTFVO denotes ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide. At ambient pressure, ρ(T) exhibits resistivity minimum near T_min ∼ 170 K followed by a gradual increase below it. With increasing pressure, T_min abruptly decreases till 4 GPa, beyond which it slightly increases. The increase ofT _min above 4 GPa is discussed in terms of the enhancement of the π-d interaction by applying pressure.     

Influence ofβ-cyclodextrin as an encapsule and as an inclusion complex dopant on conducting polyaniline

An investigation on the effect ofβ-cyclodextrin (CD) in both free and inclusion-complexed forms with a guest anionic metal complex, dioxalatodiaquochromate(III) (DDC), on the characteristics of conducting polyaniline (PANI) is carried out. Four materials, PANI (i.e. PANI-SO ₄ ²⁻ ), PANI-DDC, PANI-CD and PANI-CD + DDC were prepared byin situ chemical oxidative polymerization in aqueous H ₂SO₄ at pH 1 and subjected to electrical conductivity and spectral (IR and UV-vis bd measurements. DDC and CD when separately incorporated, reduce the conductivity of PANI by about half whilst their inclusion complex CD + DDC enhances it. Spectral characterization reveals that DDC as a dopant and CD as an encapsule exhibit their effects through adverse interaction with imine-amine N centres and benzenoid moiety of PANI. The inclusion complex CD + DDC, on the contrary, functions as a dopant by lying in between the chains and seems to promote the extended conformation of PANI chain and hence theπ -electron delocalization. Exposure of the material to methanol vapour causes a decrease in conductivity in PANI and PANI-CD while an increase in PANI-CD + DDC. This study makes explicit the distinct role of CD as an encapsule and CD + DDC inclusion complex as a dopant in altering the electrical property of PANI.     

Measurement of Cu and Zn adsorption onto surficial sediment components: new evidence for less importance of clay minerals

Clay minerals in surficial sediment samples, collected from the Songhua River in China, were separated via sedimentation after removal of Fe/Mn oxides and organic materials; Cu and Zn adsorption onto the sediment components was then evaluated. Clay minerals were examined via X-ray diffraction and scanning electron microscopy. Clay minerals were found to consist mainly of illite, kaolinite, chlorite and an illite/smectite mixed layer. Non-clay minerals were dominated by quartz and orthoclase. The retention of Cu and Zn by clay minerals was 1.6 and 2.5 times, respectively, greater than that of the whole, untreated surficial sediment. Compared to the other critical components in sediments related to metal sorption (Mn oxides, Fe oxides and organic materials), the adsorption capacity of clay minerals was found to be relatively lower on a unit mass basis. These data suggest that, although clay minerals may be important in the adsorption of heavy metals to aquatic sediments, their role is less significant than Fe/Mn oxides and organic materials.     

Magnetic and Electronic Properties of Nitrogen-Doped Lanthanum Sesquioxide La<Subscript>2</Subscript>O<Subscript>3</Subscript> as Predicted from First Principles

Using the ab initio FLAPW-GGA method, we examine the electronic and magnetic properties of nitrogen-doped non-magnetic sesquioxide La₂O₃ emphasizing the role of doping sites in the occurrence of d ⁰-magnetism. We predict the magnetization of La₂O₃ induced by nitrogen impurity in both octahedral and tetrahedral sites of the oxygen sublattice. The most interesting results are that (i) the total magnetic moments (about 1 μ _B per supercells) are independent of the doping site, whereas (ii) the electronic spectra of these systems differ drastically: La₂O₃:N with six-fold coordinated nitrogen behaves as a narrow-band-gap magnetic semiconductor, whereas with four-fold coordinated nitrogen is predicted to be a magnetic half-metal. This effect is explained taking into account the differences in N-2p_z^{ˉ -}2p_{z}^{\downarrow \uparrow} versus N-2p_x,y^{ˉ -}2p_{x,y}^{\downarrow \uparrow} orbital splitting for various doping sites. Thus, the type of the doping site is one of the essential factors for designing of new d ⁰-magnetic materials with promising properties.     

Ab initio molecular dynamics studies of metal clusters

We present results of ourab initio molecular dynamics simulations on the atomic and electronic structure of clusters of divalent metals, aluminum and antimony, which exhibit a range of bonding characteristics e.g. non-metal-metal transition, metallic and covalent respectively. Results of these studies have been used to develop icosahedral AI₁₂X (X = C, Si and Ge) superatoms with 40 valence electrons which correspond to a filled electronic shell. It is found that the doping leads to a large gain in the binding energy as compared to Al₁₃, suggesting this to be a novel way of developing species for cluster assembled materials. Further studies of adsorption of Li, Si and Cl atoms on Al₇ and Al₁₃ clusters show marked variation in the adsorption behaviour of clusters as a function of size and the adsorbate. Silicon reconstructs both the clusters and induces covalency in Al-Al bonds. We discuss the adsorption behaviour in terms of the superatom-atom interactions.     

Novel caged clusters of silicon: Fullerenes,Frank-Kasper polyhedron and cubic

We review recent findings of metal (M) encapsulated caged clusters of Si and Ge obtained from computer experiments based on an ab initio pseudopotential method. It is shown that one M atom changes drastically the properties of Si and Ge clusters and that depending upon the size of the M atom, cages of 14, 15, and 16 Si as well as Ge atoms are formed. In particular M@Si₁₆ silicon fullerene has been obtained for M= Zr and Hf, while a Frank-Kasper polyhedron has been obtained for M@X₁₆, X = Si and Ge. These clusters show high stability and large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps which are likely to make these species strongly abundant. A regular icosahedral M@X₁₂ cluster has also been obtained for X = Ge and Sn by doping a divalent M atom. Interactions between clusters are rather weak. This is attractive for developing self-assembled cluster materials.     

Theory of Plane Copper and Oxygen Nuclear Spin–Lattice Relaxation Rates in Lightly Doped La2− x Sr x CuO4

Using a Mori-Zwanzig projection operator procedure the relaxation function theory of doped two-dimensional Heisenberg antiferromagnetic (AF) system in the paramagnetic state is presented taking into account the hole subsystem as well as both the electron and AF correlations. At low temperatures the main contribution to the nuclear spin–lattice relaxation rate, ⁶³(1/T ₁), of plane ⁶³Cu, arises from the AF fluctuations, and ¹⁷(1/T ₁), of plane ¹⁷O, has the contributions from the wave vectors in the vicinity of (π,π) and small q ∼ 0. The effects of thermal spin-wave damping Γ _q on ¹⁷(1/T ₁) in lightly doped regime are investigated, suggesting either a polynomial of up to third order (not simply (T/J)³) or exponential temperature dependence of Γ _q at low temperatures. It is shown that the theory is able to explain the main features of experimental data on temperature and doping dependence of ^17,63(1/T ₁) in the paramagnetic state of La_{2− x} Sr _x CuO₄ compounds.     

Electronic Structure of Corannulene Monoanion: A Comparative Study with the Neutral and Tetraanionic States

Abstract

Electronic structure of monoanionic corannulene, C₂₀H^- ₁₀, has been studied comparing with those of C₂₀H₁₀ and C₂₀H^4- ₁₀ based on the ab initio molecular orbital calculation at the HF/3-21G level. The difference in geometries and electronic structures among these three corannulene systems and the strength of Jahn-Teller distortion appearing in C₂₀H^- ₁₀ are discussed.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

Eliminating nuisance parameters: two characterizations

Given a statistical modelP = {P_θ : θ ∈ x} and a surjective functiong: ϑ→Λ the problem of transformingP into a new modelQ= {_λ : λ ∈ Λ} indexed by Λ is investigated. Two characterizations are given for those modelsQ of the form Q_λ = ∫ P_θ π_λ(dθ), where π_λ is some probability such that π_λ(g=λ)=1. The first is related to a geometric property ofQ, while the second rests on the inferential implications of adoptingQ. Also, in the first π_λ is allowed to be finitely additive, while in the second π_λ is σ-additive. Finally, integrated likelihoods are revisited in light of the second characterization.     

A detailed model for the decomposition of nitramines: RDX and HMX

A unified decomposition scheme for two very important cyclic nitramines used as primary explosives – RDX and HMX – has been constructed using ab initio Density Functional Theory (DFT) calculations. Molecular parameters such as vibrational frequencies and moments of inertia corresponding to the computed potential energy profile of unimolecular decomposition of these nitramines were then used to obtain the thermochemistry of all identified species and reaction rate constants of each individual channel. These primary decomposition reactions were then combined with: (i) important secondary reactions of the key reactive radical intermediates, such as CH₂NNO₂ (Methylene Nitramine MN), CH₂N, NO, NO₂, OH, etc.; (ii) existing nitramie reaction networks [33]. We have developed an improved mechanism for the detailed chemistry of nitramines which can be applied to combustion and detonation phenomena of this class of energetic materials. This revised version was published online in June 2006 with corrections to the Cover Date.