First-principles approach to the calculation of electronic spectra in clusters

We discuss a method for first-principles calculations of photoemission spectra in small clusters, going well beyond a standard density functional theory-local density approximation (DFT-LDA) approach. Starting with a DFT-LDA calculation, we evaluate self-energy contributions to the quasiparticle energies of an electron or hole in the GW scheme, where the self-energy Σ = GW is constructed from the one-particle Green's function G and the RPA screened Coulomb interaction W. The contributions of structural relaxation are taken into account. We show the importance of these effects at the example of the photoemission spectrum of SiH₄. We also briefly discuss results for longer hydrogenated silicon chains, and address the problem of optical absorption.     

A comparative study of the atomic and electronic structure of F centers in ferroelectric KNbO3: Ab initio and semi-empirical calculations

The linear muffin-tin-orbital method combined with density functional theory (in a local density approximation) and the semi-empirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the supercell study of the F centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO₃ crystals. The two electrons are found to be considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the F center in KNbO₃ resembles much more electron defects in the partly covalent SiO₂ crystal (the so-called E₁′ center) rather than usual F centers in ionic crystals like MgO and alkali halides. This covalency is confirmed by the analysis of the electronic density distribution. The absorption energies were calculated by means of the INDO method using the ΔSCF scheme after a relaxation of atoms surrounding the F center. For the orthorhombic phase three absoprtion bands are predicted, the first one is close to that observed experimentally under electron irradiaton.     

叠氮甲烷二聚体分子间相互作用的abinitio研究

运用ab initio方法,在HF水平6-311G**基组下求得叠氮甲烷二聚体势能面上六种优化构型和电子结构.经MP2电子相关和基组叠加误差(BSSE)以及零点能(ZPE)校正,求得分子间相互作用能;结合能(9.49 kJ*mol-1)最大的二聚体为存在两个CHN氢键的六元环构型,属于D2h群.探讨了甲基内旋转对相互作用能的影响.由自然键轨道(NBO)分析揭示了相互作用的本质.对单体和六种二聚体优化构型进行简谐振动分析,研究了IR光谱变化规律.此外还基于统计热力学求得273.15～800.00 K温度范围从单体形成二聚体的热力学性质变化.     

Systematical study on the roles of transition alloying substitutions on anti-disproportionation reaction of ZrCo during charging and releasing hydrogen

Intermetallic alloy ZrCo is believed to be a good substitution for uranium to store tritium. Nevertheless, disproportionation reaction often happens during the hydriding and dehydriding processes, and hydrogen storage property of ZrCo is therefore degraded. Alloying elements are often used to substitute Zr or Co in ZrCo to restrain disproportionation reaction. However, many experimental results do not agree with each other, and it lacks overall tendency for all transition metal elements. In this work, systematical ab initio calculations are performed to study more than 20 transition alloying elements to substitute Co and Zr in ZrCoH₃ to study the anti-disproportionation effects. It is found that substitution of Co by transition metal elements on anti-disproportionation reaction is unconspicuous, and only Ni can enlarge Zr–H bond length and decrease the volume of 8e site, presenting anti-disproportionation effect, which qualitatively agrees with the previous experiments. In contrast, all transition alloying elements considered except Fe, Co, Ni, Ru, Rh, Pd, Os and Ir replacing Zr can both enlarge the length of Zr–H bond and decrease the volume of 8e site, and thus restrain the disproportionation effects. At last, two-dimensional charge density and density of states are calculated to analyze the underlying mico-mechanisms affecting the effects of transition alloying elements on anti-disproportionation reaction.     

Ultrastrong Medium‐Entropy Single‐Phase Alloys Designed via Severe Lattice Distortion

Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium‐ and high‐entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium‐entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid‐solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation‐mediated plasticity effectively enhances the strength–ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra‐strong materials for structural engineering applications.     

Molecular Formation upon Ionization of van der Waals Clusters and Implication to Astrochemistry

The presence and formation of a large variety of organic molecules in the interstellar medium is evident from both astronomical data of absorption and emission bands at different regions of the spectrum. Specifically, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM). The mechanism for their formation in ISM‘s low temperature environment is, as of yet, a mystery nonetheless. Understanding the mechanism of formation of complex molecules such as PAHs and nitrogen-based PAHs (PANH) in the ISM is a long-standing challenge which has been drawing a growing attention for the past several decades. In this review we wish to emphasize two things: Firstly, the essential role quantum chemistry can play in the study of astrochemical reactions. Secondly, we wish to demonstrate that said variety of possibilities for chemical reaction, starting upon ionization of van der Waals clusters. The potential for different chemical reactions to occur within a cluster environment arises from the fact that such processes can take place at low temperatures as the systems pose large amounts of energy upon ionization. Moreover, the spectator molecules in the cluster can provide a dissipation route for energy by detachment from the cluster, thus the system can stabilize efficiently even at low densities. The spectator molecules can also change the potential energy surface, by which it will pose a catalytic effect for certain reactions. We will demonstrate this by presenting Ab Initio Molecular Dynamic results on ionization of small acetylene clusters.     

DFT investigation analyzed with data mining technique of rare-earth dihydrides REH2 for hydrogen storage

Ab initio calculation and data mining are very useful methods to predict the interrelationships among structure properties. In this paper, the first-principles modelling based on the density functional theory (DFT) was used to explore the structural, mechanical, and electronic properties of ten hydrides REH₂ (RE = La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm). From the present work, it can be noted that LaH₂, CeH₂, NdH_2, and PrH₂ compounds crystallizing in the hydrogen-rich cubic phase present good mechanical properties which could be of use to future applications in hydrogen storage. Furthermore, the electronic structures were also evaluated to provide a deep insight regarding their trends and confirm the metallic character of these systems. While, HoH₂, DyH₂, ErH₂, TbH₂, TmH₂, SmH₂ hydrides are mainly hard and rigid systems with high values of the bulk modulus (B), shear modulus (G), Young's modulus (E), micro-hardness (H), and low values of B/G ratio. More significantly, the adopted principal component analysis (PCA) provided an advantageous technique in selecting and identifying the minimum of inputs variables necessary to capture all information of the systems.     

First Principles Analysis of the Electrocatalytic Oxidation of Methanol and Carbon Monoxide

The strong drive to commercialize fuel cells for portable as well as transportation power sources has led to the tremendous growth in fundamental research aimed at elucidating the catalytic paths and kinetics that govern the electrode performance of proton exchange membrane (PEM) fuel cells. Advances in theory over the past decade coupled with the exponential increases in computational speed and memory have enabled theory to become an invaluable partner in elucidating the surface chemistry that controls different catalytic systems. Despite the significant advances in modeling vapor-phase catalytic systems, the widespread use of first principle theoretical calculations in the analysis of electrocatalytic systems has been rather limited due to the complex electrochemical environment. Herein, we describe the development and application of a first-principles-based approach termed the double reference method that can be used to simulate chemistry at an electrified interface. The simulations mimic the half-cell analysis that is currently used to evaluate electrochemical systems experimentally where the potential is set via an external potentiostat. We use this approach to simulate the potential dependence of elementary reaction energies and activation barriers for different electrocatalytic reactions important for the anode of the direct methanol fuel cell. More specifically we examine the potential-dependence for the activation of water and the oxidation of methanol and CO over model Pt and Pt alloy surfaces. The insights from these model systems are subsequently used to test alternative compositions for the development of improved catalytic materials for the anode of the direct methanol fuel cell.     

S2分子的激发态与X3Σ-g→B3Σ-u吸收谱研究

S₂分子是一种在能源和激光领域有应用前景的物质,几十年来,人们对S₂分子的电子结构与光谱的研究不断深入。应用量子化学从头算方法(完全活性空间多组态自洽场方法和多参考二阶微扰方法)计算了S₂分子的11个电子态,并与其他文献结果作了比较,得到了与其他文献符合较好的结果;讨论了不同方法对激发态计算的适用性的问题:完全活性空间多组态自洽场方法适用于分子平衡态几何构形的优化,考虑到电子的动力学相关,能量的计算多参考二阶微扰方法更好。计算了S₂分子X³Σ^-_g(v)→B³Σ^-_u的垂直吸收谱,给出了速度规范下基态电子态各振动级X³Σ^-_g(v)垂直跃迁到B³Σ^-_u的振子强度,结果表明S₂分子是一种泵浦范围较宽(350~600 nm)的良好的激光工作物质。     

Insights from ab initio molecular dynamics simulations for a multicomponent oxide glass

It remains a challenge to establish structural models of multicomponent oxide glass systems. In this study, we have investigated 68.3SiO₂–16.1B₂O₃–4.2Al₂O₃–11.4Na₂O glass and melt structures by ab initio molecular dynamics (AIMD) simulations. The atomic configurations obtained from AIMD simulations were validated against ¹⁷O solid‐state NMR spectrum under 24.0 T and neutron diffraction data, and excellent agreement was achieved. The bond lengths, angles, and coordination geometries were statistically analyzed for each atomic species. Here we particularly address the role of minor atomic species such as five‐coordinate Si (Si^V) and Al (Al^V). The Si^V–O bond lengths and O–Si^V–O angle distribution in the glass indicated 1.718 Å and three peaks at 90°, 120°, and 175°, which are assigned to a coordination geometry of the trigonal bipyramidal structure. Ring statistic analysis revealed that Si^V and Al^V were found to preferentially contribute to the formation of small ring sizes.