Higher-energy Structures and Stability of Cu and Al Crystals Along Displacive Transformation Paths

Stability of ground-state and higher-energy phases of Cu and Al encountered along the tetragonal (bcc–fcc), trigonal (bcc-simple cubic-fcc) and hexagonal (bcc–hcp) displacive transformation paths is studied by ab initio electronic structure calculations (ultra-soft pseudopotentials, VASP code) and by many-body semi-empirical interatomic potentials developed by Mishin et al. Comparison of these two calculations provides a means for further analysis of the efficacy of the potentials in atomistic modeling of more complicated configurations, such as extended defects.     

晶格反演法构建SiC/Mg界面对势及其验证

通过分析SiC和Mg的晶体结构,构造了SiC与Mg的共格晶面。采用Chen-M9bius晶格反演法对SiC/Mg界面的原子对势进行了反演,分别推导了Si和Mg原子、C和Mg原子间作用势与界面结合能关系的解析表达式。在此基础上,从头计算了SiC/Mg界面的结合能曲线,结合反演公式得到了Si和Mg原子、C和Mg原子的对势函数,并对反演过程的自洽性进行了检验。此外,建立了其他6种不同的SiC/Mg界面构型,通过对比反演对势计算的界面结合能与从头计算的结果,对反演所得对势的可转移性进行了验证。结果表明:从头计算的界面结合能可以由反演对势精确地算出,整个反演过程和结果是自洽的,所得原子对势同样适用于其他界面构型,具有良好的可转移性。本文推导的反演公式同样适用于与SiC/Mg界面结构相似的其他界面原子对势的研究。     

Au纳米粒子的束缚能

利用从头计算法计算了一定粒径下Au纳米量子点的谐振子束缚能的大小。根据对波函数的分析发现,谐振子势是描述量子点的一个较好的势模型。     

蛋白质结构预测的优化模型与方法

从头预测方法是一种主要的蛋白质空间结构预测方法,其核心内容是恰当地建立并求解一个复杂的全局优化问题,40年来,虽然也取得了许多研究成果,但该问题的研究始终没有克服两个方面的困难,即如何找到一个表征蛋白质结构与能量关系的势能函数和一种有效的全局优化方法,主要介绍了蛋白质结构预测的优化模型和方法。     

Elementary Processes of AlN Formation from the AlCl3 + NH3 Reaction:Transitional Structures Optimization and Thermodynamic Calculation

Elementary processes of AlCl3 ammonolysis havebeen studied theoretically by quantum chemistry methods.The structures of these transitional products involved were optimized at B3LYP, HF and MP2 level, which implemented in the Guassian98 program package. For an accurate estimation of thermodynamic properties of ammonolysis, the entropies, Gibbs free energies and reaction rates of reactions have also been taken into account.     

Molecular dynamics simulations of fluid carbon dioxide using the model potential based on ab initio MO calculation

The nonbonding interaction energy of carbon dioxide dimer was calculated using several basis sets to evaluate the basis set effect. Large basis sets including diffuse polarized functions were necessary to evaluate the interaction energy correctly. Small basis sets considerably underestimate the interaction energy. Nonbonding parameters of a model potential of carbon dioxide were refined based on the interaction energies of 40 geometrical configuration dimers calculated by an MP2/6-311 + G(2df)-level ab initio method with the BSSE correction. The molar volume and heat of evaporation of liquid carbon dioxide obtained from molecular dynamics simulations with the model potential reproduced the experimental values with only a few percentage errors. On the other hand the model potential based on the BSSE uncorrected interaction energies overestimated the attractive interaction and failed to reproduce the experimental values. The same model potential also reproduced the experimental pressure and self-diffusion coefficient of super critical fluid satisfactorily.     

Point defects in ZnO: an approach from first principles

Abstract

Recent first-principles studies of point defects in ZnO are reviewed with a focus on native defects. Key properties of defects, such as formation energies, donor and acceptor levels, optical transition energies, migration energies and atomic and electronic structure, have been evaluated using various approaches including the local density approximation (LDA) and generalized gradient approximation (GGA) to DFT, LDA+U/GGA+U, hybrid Hartree–Fock density functionals, sX and GW approximation. Results significantly depend on the approximation to exchange correlation, the simulation models for defects and the post-processes to correct shortcomings of the approximation and models. The choice of a proper approach is, therefore, crucial for reliable theoretical predictions. First-principles studies have provided an insight into the energetics and atomic and electronic structures of native point defects and impurities and defect-induced properties of ZnO. Native defects that are relevant to the n-type conductivity and the non-stoichiometry toward the O-deficient side in reduced ZnO have been debated. It is suggested that the O vacancy is responsible for the non-stoichiometry because of its low formation energy under O-poor chemical potential conditions. However, the O vacancy is a very deep donor and cannot be a major source of carrier electrons. The Zn interstitial and anti-site are shallow donors, but these defects are unlikely to form at a high concentration in n-type ZnO under thermal equilibrium. Therefore, the n-type conductivity is attributed to other sources such as residual impurities including H impurities with several atomic configurations, a metastable shallow donor state of the O vacancy, and defect complexes involving the Zn interstitial. Among the native acceptor-type defects, the Zn vacancy is dominant. It is a deep acceptor and cannot produce a high concentration of holes. The O interstitial and anti-site are high in formation energy and/or are electrically inactive and, hence, are unlikely to play essential roles in electrical properties. Overall defect energetics suggests a preference for the native donor-type defects over acceptor-type defects in ZnO. The O vacancy, Zn interstitial and Zn anti-site have very low formation energies when the Fermi level is low. Therefore, these defects are expected to be sources of a strong hole compensation in p-type ZnO. For the n-type doping, the compensation of carrier electrons by the native acceptor-type defects can be mostly suppressed when O-poor chemical potential conditions, i.e. low O partial pressure conditions, are chosen during crystal growth and/or doping.     

Determination of creep parameters from indentation creep experiments: a parametric finite element study for single phase materials

Abstract

This paper explores the possibilities of determining creep parameters for a simple Norton law material from indentation creep testing. Using creep finite element analysis the creep indentation test technique is analysed in terms of indentation rates at constant loads. Emphasis is placed on the evolving stress distribution in front of the indenter during indentation creep. Moreover the role of indenter geometry, size effects and of macroscopic constraints is explicitly considered. A simple procedure is proposed to translate indentation creep results into constitutive creep equations for cases where the dimensions of the tested material are significantly larger than the indenter. The influence of macroscopic constraints becomes important when the size of the indenter is of the same order of magnitude as the size of the testing material. As a striking example for size effects and for macroscopic constraints the indentation creep process in a thin film is analyzed. The results contribute to a better mechanical understanding of indentation creep testing.     

Evaluation of the dielectric parameters from TSDC spectra: application to polymeric systems

The Thermally Stimulated Discharge Current (TSDC) technique is widely used for the study of main and secondary dielectric relaxations in polymers. The TSD current is described by different equations that can be arranged in a unique three-parameters (the activation energy W, A and B) general form. The physical meaning of A and B depends on the origin of the discharge currents. In this paper a method is proposed to obtain these parameters by fitting the experimental data with the analytical expression of the current, in the range around the maximum. Simulations were carried out to underline the relative importance of the parameters. A method is proposed for the decomposition of experimentally determined complex bands into a limited number of elementary peaks, each of them characterized by average values for W and B. The errors resulting from different approximations used in the analytical current expression or by the utilization of various expressions for the relaxation time are analyzed. The method is applied for the analy-sis of the TSDC spectra in the glass-rubber transition temperature regions of PET and PMMA, yielding several peaks characterized by narrow distributions of W (ΔW≈± 0.06 eV). Received: 18 September 2000 / Reviewed and accepted: 20 September 2000