Density functional theory study of magnetic coupling between Cu atoms and aniline molecules

To study the influence of transition metal atoms on the magnetic properties of compounds based on aniline and aminonaphthalenesulfonic acid, we explore theoretically a simple model based on stacked aniline molecules (radicals) and intercalated Cu atoms. Calculations were performed within the non-local spin density approximation with Becke's exchange functional and the gradient-corrected functional of Lee, Yang and Parr (BYLP) using the DMol program. The results reveal a surprisingly strong interaction between the Cu s and d valence electrons with the molecular states, leading to a ferromagnetic coupling in the case of Cu0 and CuII with molecular aniline. In the case of aniline radicals, the coupling is ferromagnetic for Cu0 and ferrimagnetic for CuII. Stacks involving three and four aniline molecules show localization of the spin density to the molecules adjacent to the Cu atom.     

Density functional methods as computational tools in materials design

Summary This article gives a brief overview of density functional theory and discusses two specific implementations: a numerical localized basis approach (DMol) and the pseudopotential plane-wave method. Characteristic examples include Cu, clusters, CO and NO dissociation on copper surfaces, Li-, K-, and O-endohedral fullerenes, tris-quaternary ammonium cations as zeolite template, and oxygen defects in bulk SiO₂. The calculations reveal the energetically favorable structures (estimated to be within ± 0.02 Å of experiment), the energetics of geometric changes, and the electronic structures underlying the bonding mechanisms. A characteristic DMo1 calculation on a 128-node nCUBE 2 parallel computer shows a speedup of about 107 over a single processor. A plane-wave calculation on a unit cell with 64 silicon atoms using 1024 nCUBE 2 processors runs about five times faster than on a single-processor CRAY YMP.     

Density functional study of the ionization potentials and electron affinities of small Nin clusters with n=2–6 and 8

The ionization potentials and electron affinities of Ni_n clusters, with n=2–6 and 8, are studied in this work using the generalized gradient approximation to the density functional theory. Important changes observed after geometry and spin multiplicity optimization suggest that the ionization processes under study cannot be simply described as the result of the removal or addition of an electron from or to the corresponding neutral system. Our results indicate that both ionization processes mainly involve electrons having s or sp dominant characters. Experimental ionization potentials are very well reproduced by the present theoretical results. The experimental electron affinities, on the other hand, are systematically underestimated by the present calculations although the overall trend is well reproduced.     

半导体非线性光学材料的第一性原理研究

通过第一性原理研究Ⅱ-Ⅳ-Ⅴ2族CKP半导体中的CdSiAs2，计算了其双折射性，量化了双折射性同应力的线性关系，它的负双折射性使之能够通过应力、温度调节以及同CdGeAs2混合来设计非临界位相匹配材料。计算显示，少量的参杂Ge（ 〈5％），能够实现非临界位相匹配I类二次谐波产生（SHG）在CO2激光谱线范围可调谐，它可能具有很高的有效χ^（2）。     

Interfacial interactions in clay-based nylon 6 nanocomposites: A density functional theory study

Density functional theory was used to study the interfacial interactions between clay and polymer in polymer/clay nanocomposites, with a focus on nylon 6 matrix. The binding energy and the distance between nylon 6 and clay surface were predicted. The effect of the isomorphic substitution in clay octahedral and tetrahedral layer on the strength of the interfacial interactions was also examined. The interaction strength between nylon 6 and clay surface was found to increase with the degree of isomorphic substitution. And the magnitude of the binding forces, reflected from the calculated binding energies, was found to be higher when the substitution took place in tetrahedral layer (e.g., Al³⁺ for Si⁴⁺). No covalent bonds were observed between nylon 6 and the clay, which means that the chemical structures of the clay and nylon 6 are unchanged during the mixing process.     

Mechanical and electronic properties of C60 under structure distortion studied with density functional theory

B3PW91/6-31G density functional method was employed to investigate the elastic, strength and electronic properties of C₆₀(I_h) in its ground electronic state (X ¹A_g). Most of the properties were examined for larger structure distortions. The over-all elastic constant were derived from the near-equilibrium potential energy curves (PECs) in five independent directions [according to symmetries by 1. D_5d, 2. D_3d, 3. D_2h, 4. C_2h(1), 5. C_2h(2)]. By extension of the distortions as large as the structure of C₆₀ was destroyed, the necessary energies were obtained, which quantitatively illuminated the stability of C₆₀ theoretically. Analytical polynomial fit of the full PECs reproduced the energy data accurately. Time-dependent B3PW91/6-31G analysis showed significant electronic spectra changes associated with the structure distortions. Elongation in the direction of D_5d and compression in that of D_2h encountered potential energy surface cross-linkages, which might be considered as a single electron pump for further application in designs of single electron devices.     

Structural and electronic properties of Ti-nanowires/C-single wall nanotubes composites by density functional theory calculations

Structural and electronic properties of composite Ti-nanowires/single wall carbon nanotubes ((6,0) and (10,0)) (SWNT) were evaluated by means of density functional theory computations. We considered the cases of monoatomic (MNW), BCC (β-NW) and HCP (α-NW) nanowires that were either inserted or deposited in/on the SWNTs. In all cases the NWs turn the cylindrical SWNTs’ shape to ellipsoid, an effect that is closely related to charge transfer from Ti toward C neighboring atoms. We found that the wires inside the SWNT appear to be more stable compared to the outside cases, while all NWs contribute with new energy states at the Fermi level, transforming the semiconducting (10,0) to a conducting composite. In addition, we found spin up–down differences in the β-NW_on case and electronic charge redistributions e.g. in α-NW_in (charge accumulation internally along the tube's axis) or in α-NW_on (superficial charge accumulation in the vicinity of the NW), accompanied by manifestation of electric dipole moment that reaches the value of 10 Debye in a-NW_on. These results may be of use in the design of new C-based nanocomposite systems suitable for applications in microelectronics, sensors and catalysis.     

Interstitial and substitutional zirconium in SrTiO3

We investigate Zr in SrTiO₃ (STO) as a model for nuclear waste forms in which the fission product ⁹⁰Sr eventually decays to stable Zr through beta emission. The transformation of a divalent into a tetravalent constituent is expected to affect the long-term structural and chemical stability of this solid. Computational methods of electronic structure theory, specifically the density functional theory (DFT) within the supercell model, are used to predict the thermodynamic stability and electronic states of interstitial and Sr- or Ti-substituted Zr atoms in the STO lattice. Native defects such as vacancies and antisites are also considered. When Zr replaces Sr, its most stable configuration is to simply occupy the Sr site. For Zr added to the lattice, its most stable configuration is to replace a Ti, making a Zr_Ti impurity plus a Ti interstitial. Zr_Sr is predicted to be a double electron donor, Zr_Ti is electrically inactive and interstitial Zr and Ti are predicted to be quadruple donors, with all donor levels in the conduction band. The interstitials are all predicted to increase the crystal volume, and lead to a tetragonal distortion of the lattice. Experiments with injection of Zr and O atoms into STO qualitatively confirm these predictions of crystal structural changes.     

Structural evolution of clusters using genetic algorithm and density functional theory method

Structural evolution of clusters have been studied using an extensive, unbiased search based on genetic algorithm and density functional theory (DFT) methods. Cationic, neutral, and anionic silver clusters have planar shapes for their lowest-energy structures up to n = 7, 6, and 6, respectively. Most of the competitive candidates for are found to adopt close-flat configurations. The present results obtained by employing the Perdew–Wang 91 (PW91) exchange-correlation functional are significantly different from those predicted in earlier work using empirical and semi-empirical potentials, and partly in line with the previous first-principles calculations. The dependences of the lowest-energy structures of on second finite differences of total energy, binding energies per atom, highest occupied and lowest unoccupied molecular orbital energy gaps, ionization potentials, and electron affinities are studied in detail. The calculated ionization potentials and electron affinities of the optimal clusters display distinct even–odd oscillations. The neutral Ag clusters with 6-, 8-, and 14-atoms are suggested to be “magic” clusters by an analysis of their geometric and electronic properties.     

Magnetic properties of molecular systems: A nonlocal spin density functional study

A new class of molecular magnets based on aniline and aminonaphthalene sulfonic acid was previously reported. In the present work, a nonlocal density functional theory study was performed on simple molecular models, to determine the mechanism that stabilizes the parallel alignment of the spin component. The role played by transition metals on the magnetic properties of this kind of systems was also studied within this approximation. The results obtained show a high sensitivity of the magnetic coupling on the molecular geometry as well as on the nature and oxidation state of the metallic atom.     

Density functional theory study of alloy element interstitials in Al

Density Functional Theory calculations were used to study Mg, Si, Cr, Mn, Fe, Co, Ni, and Cu interstitial configurations in Al. Energies of these elements in (100) dumbbell and octahedral configurations were determined. Results show that it is energetically favourable for metal alloying element atoms to replace Al selfinterstitials if the alloying atoms are smaller than the Al atoms, as expected. The system energy can thus be decreased by up to 2 eV. The difference between the energies of (100) dumbbell and octahedral configurations is only a few tenth eV for the alloys with metallic alloying elements. For Si, the difference can be up to 0.9 eV. This exceptional behavior of Si is most likely due to its angularly dependent bonding characteristics. Short ab-initio Molecular Dynamics simulations were performed on Mg and Si interstitials to allow these systems to evolve into different interstitial configurations rather than just the (100) dumbbell and octahedral configurations. For Si an alternative configuration with tetrahedral-like coordination was found. Consequences of the calculation results for radiation-induced segregation are discussed.     

Selective adsorption of cations on single-walled carbon nanotubes: A density functional theory study

The selective adsorption of cation on single-walled carbon nanotubes (SWNTs) is systemically studied by using density functional theory calculations. It is found that the adsorption energy of cations on SWNTs depends on the concentration of cations and the diameter and the electronic structure of SWNTs. The binding strength of on each SWNT increases monotonically as the concentration of decreases, undergoing a change from endothermic to exothermic reaction. Generally speaking, the binding of on SWNTs becomes weaker as the diameter increases. In the medium-diameter region (9 < d < 11 Å), prefers to interact with metallic SWNTs (m-SWNTs) rather than semiconducting SWNTs (s-SWNTs) at the same concentration of . In the small-diameter region (d < 9 Å), the binding of is nearly independent of metallicity, but it is stronger than that of on the medium-diameter s-SWNTs. In the large-diameter region (d > 11 Å), the dependence of adsorption on the electronic structure is complicated, but the binding of is weaker than that on the medium-diameter s-SWNTs. Our results are in agreement with the experimental report that the small-diameter m- and s-SWNTs and the medium-diameter m-SWNTs are etched away by while the medium-diameter s-SWNTs and the large-diameter m- and s-SWNTs are intact.     

浅析语符学理论中的功能理论

叶尔姆斯列夫是哥本哈根学派的主要代表人物，其语符学理论是哥本哈根语符学派理论的基础，而其中的功能理论又是语符学理论的重要部分。本文旨在分析叶尔姆斯列夫的语言功能理论，以期读者能更加全面和深刻地理解语符学理论和哥本哈根学派。     

A theoretical study on initial growth mechanism of ZrO2 film using cyclopentadienyl-type precursor

The initial growth reaction of atomic layer deposition (ALD) of ZrO₂ using Cp₂Zr(CH₃)₂ (CpC₅H₅, cyclopentadienyl) as metal precursor on the hydroxylated silicon surface is investigated by using density functional theory (DFT). The ALD cycle is achieved through two types of ligand elimination reactions (i.e., CH₄ and CpH elimination reactions). The possible reaction pathways are proposed in order to find the dominant initial reaction during the Cp₂Zr(CH₃)₂ precursor pulse. DFT calculations show that the CH₄ elimination reaction is energetically more favorable than CpH elimination reaction. As a result, the two CH₃ ligands of Cp₂Zr(CH₃)₂ may be dissociated prior to the two Cp rings during the metal precursor pulse. In addition, one CpH elimination may occurs sequentially following the first CH4 elimination reaction according to activation barrier analysis during the Cp₂Zr(CH₃)₂ pulse. All the calculated results are in agreement with the experimental findings.     

First-principles study of the (001) and (110) surfaces of superhard ReB2

Structural relaxations, electronic properties, and surface energies of ReB₂ (001) and (110) surfaces with various terminations are investigated with a first-principles method. It is found that the surface interatomic spacings of ReB₂ (001) and (110) surfaces are different from those of the bulk structure. The vertical spacings between the first and second layers of the studied surfaces are contracted. The (001)-Re surface is likely to be stable without introducing a large relaxation. Among these surfaces, only the (110) surface has surface rumpling, and the Re atoms on its first layer are apt to move inward. After atomic relaxation, some covalent bonds formed by the outmost atoms of the relaxed surfaces are shorter than those of the bulk system, which indicates that the covalent B-B and Re-B bonds of the surface layer have been strengthened. An analysis of surface energies shows that after relaxation, the (001)-Re surface is more stable than other types of surfaces.     

Ab initio tensile testing simulation of aluminum and aluminum nitride ceramics based on density functional theory

The ideal strength and mechanical behavior of aluminum, aluminum nitride, and their composites at zero temperature are investigated using ab initio norm conserving pseudopotential methods based on local density approximation. The total energy and stress of supercells which contain these atomistic models are calculated under uniaxial tensile strain. Estimated various physical properties of aluminum and aluminum nitride at equilibrium state are in good agreement with the experiment. The tensile test of the composite model shows a failure of the composite in the aluminum matrix. The ideal strength of these models is also estimated.