氘、氚代聚苯乙烯单体abinitio研究

用HF,MP2及BLYP从头算方法,在6—31G基组水平上,优化了聚苯乙烯单体基态的平衡几何构型.主要用BLYP6—31方法研究了氘、氚代聚苯乙烯单体(DPS,PST)的正则振动频率、红外光谱强度、C—D键键能,并对正则振动模式进行了简单分析,同时研究了DPS,PST单体中温度、压强与熵的关系. 关键词： 平衡几何构型 正则振动频率 红外光谱强度 键能     

Ab initio study of the photodissociation of ammonia

An ab initio SCF and CI treatment of the electronic spectrum of ammonia in both the pyramidal and planar conformation is reported which employs an [8, 6, $\text{1}\text{4}$, 1] AO basis of near Hartree-Fock quality; the ground state CI energy obtained for the equilibrium conformation is ?56.4241 a.u. In addition, further calculations have been carried out at the SCF level to study various photodissociation reactions of NH₃. The calculated CI transition energies are seen to agree with corresponding experimental values to within 0.0–0.3 eV, usually in the 0.1-eV range. Photodissociation to the $\text{NH}{}_2\text{(}{}^2\text{B}\text{?}{}_1\text{) +}\text{H}\text{(}{}^2\text{S)}$ products is confirmed thereby to proceed via the $\text{A}\text{?}\text{1a″}{}_2\text{→ 3s}$ state of ammonia, but contrary to earlier speculation it is found that the transformation between reactant and products is already satisfactorily described at the SCF or orbital level, i.e., a Rydberg 3s of NH₃ is seen to be gradually converted into a pure hydrogenic 1s species as dissociation proceeds. In addition the photolysis of NH₃ to NH₂$\text{(}{}^2\text{A}{}_1\text{) +}\text{H}\text{(}{}^2\text{S)}$ is argued to occur via the $\text{C}\text{?}\text{1a″}{}_2\text{→ 3pz}{}^1\text{A′}{}_1$ state and as such is seen to be a symmetry allowed process, in contrast to the previous assignment involving the $\text{B}\text{?}\text{1a″}{}_2\text{→ 3px}$, $\text{y}{}^1\text{E″}$ species. Finally an attempt is made to analyze the mechanisms of various NH + H₂ photodissociation processes with the help of SCF calculations and symmetry arguments for various higher-lying excited states of ammonia.     

Ab initio study of the polymerisation of cyclopentasilane

The molecular structures and vibration modes of cyclopentasilane (Si₅H₁₀) have been examined by employing ab initio and density-functional methods. Three different structures of Si₅H₁₀ with different symmetries are analysed, and the results show that the envelope (C _s ) and the twist (C₂) forms have similar energies and that the planar form (D_5h ) is about 50 meV less stable than the C _s and C₂ forms. The excited-state potential energy surface of Si₅H₁₀ is performed using the CIS electronic energy calculation. The ring-open reaction of Si₅H₁₀ is investigated in detail by using the first-principles molecular-dynamics simulation for screening the reaction pathways. The formation of Si–H–Si is found to play an important role in the ring-open reaction.     

Ab initio study of graphene on SiC

Employing density-functional calculations we study single and double graphene layers on Si- and C-terminated 1x1-6H-SiC surfaces. We show that, in contrast with earlier assumptions, the first carbon layer is covalently bonded to the substrate and cannot be responsible for the graphene-type electronic spectrum observed experimentally. The characteristic spectrum of freestanding graphene appears with the second carbon layer, which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, the interface is metallic, whereas on C face it is semiconducting or semimetallic for single or double graphene coverage, respectively.     

Ab initio study of SrTiO3 surfaces

We present first-principles total-energy calculations of (001) surfaces of SrTiO₃. Both SrO-terminated and TiO₂-terminated surfaces are considered, and the results are compared with previous calculations for BaTiO₃ surfaces. The major differences are in the details of the relaxed surface structures. Our calculations argue against the existence of a large ferroelectric relaxation in the surface layer, as had been previously proposed. We do find some indications of a weak surface ferroelectric instability, but so weak as to be easily destroyed by thermal fluctuations, except perhaps at quite low temperatures. We also compute surface relaxation energies and surface electronic band structures, obtaining results that are generally similar to those for BaTiO₃.     

Ab initio study of PrAg intermetallic compound

In this work, a first-principles study on PrAg compound using the density functional theory implemented in the projector-augmented wave (PAW) method in the CsCl (B2) crystal structure has been performed. Based on the optimized structural parameter, which is in good agreement with experimental data, the electronic structure, elastic, thermodynamics and vibrational properties have been investigated. The temperature and pressure variations of volume, bulk modulus, thermal expansion coefficient, heat capacities, and Debye temperatures in wide pressure (0-30 GPa) and temperature ranges have also been predicted.     

Ab initio study of the distribution of point defects at grain boundaries in crystalline silicon

The segregation of a vacancy and phosphorus on inclined grain boundaries in crystalline silicon has been calculated. It has been found that the distribution of defects at boundaries under study depends both on the nature of a defect and on the local structure of the boundary. The parameters characterizing the local deformation of the tetrahedral environment of an atom at the boundary have been proposed. A linear correlation has been found between the energy of the distribution and proposed parameters.     

Ab initio study of the structural properties of magnesium

Structural properties of magnesium in the hcp structure are calculated using the ab initio pseudopotential method within the local-density-functional formalism. The calculated lattice constants, cohesive energy, bulk modulus, and Poisson's ratio are in good agreement with experimental values. A comparison with the results of beryllium is also discussed.     

Ab initio calculation of 14N nuclear quadrupole coupling constants

The use of ab initio Hartree-Fock electric field gradient calculations to predict nitrogen nuclear quadrupole coupling constants is examined using basis sets of split valence to triple zeta plus polarization size. From results on 20 to 35 molecules for each basis, such calculations are shown to be of predictive value if systematic errors are eliminated by using regression-derived scaling factors, and the reliability of each basis is assessed. For field gradient calculations on larger molecules, a significantly better alternative to a minimal basis which adds little extra computational cost, is proposed.     

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 study of palladium and silicon carbide

Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at five charged states within the framework of density functional theory using the local spin density approximation. It was found that Pd interstitials and substitutionals have similar energy to their intrinsic counterparts. In addition, Pd substitutes for a vacancy, di-vacancy, and tri-vacancy with similar energies. Pd diffuses through SiC via an interstitial mechanism employing the tetrahedral sites and Pd can substitute for Si and C at positive charged states. Removing electrons (p-type doping) from SiC lowers the formation and migration energies of Pd defects in SiC for most configurations.     

Ab initio study of gold-doped zigzag graphene nanoribbons

The electronic transport properties of zigzag graphene nanoribbons (ZGNRs) through covalent functionalization of gold (Au) atoms is investigated by using non-equilibrium Green’s function combined with density functional theory. It is revealed that the electronic properties of Au-doped ZGNRs vary significantly due to spin and its non-inclusion. We find that the DOS profiles of Au-adsorbed ZGNR due to spin reveal very less number of states available for conduction, whereas non-inclusion of spin results in higher DOS across the Fermi level. Edge Au-doped ribbons exhibit stable structure and are energetically more favorable than the center Au-doped ZGNRs. Though the chemical interaction at the ZGNR–Au interface modifies the Fermi level, Au-adsorbed ZGNR reveals semimetallic properties. A prominent qualitative change of the I–V curve from linear to nonlinear is observed as the Au atom shifts from center toward the edges of the ribbon. Number of peaks present near the Fermi level ensures conductance channels available for charge transport in case of Au-center-substituted ZGNR. We predict semimetallic nature of the Au-adsorbed ZGNR with a high DOS peak distributed over a narrow energy region at the Fermi level and fewer conductance channels. Our calculations for the magnetic properties predict that Au functionalization leads to semiconducting nature with different band gaps for spin up and spin down. The outcomes are compared with the experimental and theoretical results available for other materials.     

黑索金的电子结构和热解机理的从头算研究

利用从头算梯度修正密度泛函理论,我们在B3LYP/6-31G(d,p)水平上,计算了高能炸药黑索金(RDX)及其热解中间产物C3H6N5O4和NO2等的电子结构、能量、键级和一些热力学性质,并在此基础上,计算了键离解能和反应速率常数.此外,我们还分别从键级和键离解能角度,讨论了RDX的热解机理,所得结果与实验一致.     

Ab initio study of isolated chlorine defects in cubic SiC

The electronic properties and formation energy of isolated Cl defects in SiC were investigated by first-principles calculations. Chlorine was studied in a substitutional position, in either a carbon (Cl(C)) or a silicon site (Cl(Si)), and in two interstitial positions (Cl(i)), either tetrahedral or octahedral configurations. Our calculations revealed that Cl(C) is energetically favored and it is a likely candidate to explain the nature of the experimentally observed Cl incorporation reported in SiC epilayers grown by chloride-based chemical vapor deposition.     

Ab initio study of shear strain effects on ferroelectricity at PbTiO3 thin films

Ferroelectric thin film with the perovskite ABO₃ structure have been widely used in technology applications, e.g., actuators in MEMS/NEMS and nonvolatile random access memories (FeRAM). In order to clarify the effect of the shear strain on the ferroelectricity, the PbTiO₃ thin film as a typical one is chosen. The focus of this study is to put on the PbO-terminated (1? × ?1) and c(2? × ?2) surfaces and the TiO₂-terminated (1? × ?1) surface. Based on ab initio density functional theory calculations with the local density approximation, we have found out that in both the PbO and TiO₂-terminated (1? × ?1) models, the ferroelectricity in the PbO layers was enhanced under the positive shear strain while it was suppressed under the negative one. For the TiO₂ layers, the ferroelectricity was slightly enhanced and sharply suppressed under the positive and negative shear strains, respectively. In the PbO-terminated (2? × ?2) model, the AFE phase was suppressed by the FE phase under the positive shear strain while the opposite trend was found under the negative shear strain. For the PbO layers, the ferroelectricity was enhanced under the positive and negative shear strains. For the TiO₂ layers, the influence of the negative shear strain on the ferroelectricity was larger than that of the positive one. In addition, the ideal strength of the PbTiO₃ thin film with the different terminations was investigated as well.