基于原子晶体构型的高压固氩中的多体相互作用

X射线衍射实验显示固氩是面心立方(fcc)晶格结构,目前对晶体氩的研究只限于两体,三体以及四体相互作用势.本文利用多体展开方法和超分子单、双(三)重激发耦合簇理论(CCSD(T))对固氩fcc晶格结构的三体和四体的几何构型、几何参数、不同体积下所有三体和四体构型的势能以及各构型所占比例等几个方面进行了准确的量子化学计算.结果表明:所有三体构型中对总的三体势能贡献最大的是构型1、构型6、构型12和构型23;三体势及其交换部分和色散部分的计算结果与现有解析经验势在长程部分符合得非常好,但在短程部分有较小差异.所有的四体构形中对总的四体势能贡献最大的是构型1,构型2,构型4,构型5,构型7和构型8;四体势及其交换势部分和色散部分的计算结果尚无解析经验势可比较.利用这些特殊构型的相关数据并结合其它构型,可拟合出更准确的三体经验势函数及其参数,也为拟合四体经验势函数及其参数提供了重要的参考价值.     

α-B晶体的Lennard-Jones对势和对势型多体势构建

精确原子间相互作用势函数的建立是分子动力学模拟的核心. 针对α-B晶体(R3m群), 分别构建Lennard-Jones (L-J)型对势和对势型多体势, 这两类势函数的构建仅需考虑晶体的原子平均结合能和几何构型参数. 前者取最近邻原子间距的势函数为最小值; 后者采用负指数(L-J型)函数和分段插值多项式形式来分别构造势函数, 在近邻处为势能极小值, 相邻极小值位置的中点引入势垒. 针对完整的α-B 晶体和偏离完整α-B晶体结构, 首先利用第一性原理计算结果来确定势函数的参数, 然后借助分子动力学的能量最小化方法优化结构, 并对这两类势函数以及Tersoff 势函数进行比较. 结果表明: L-J对势和Tersoff势的结果与α-B晶体构型有较大偏离; 而对势型多体势, 无论初始晶体构型完整与否, 其结果与完整α-B晶体构型比较一致.     

高温高密度条件下氮分子三体关联效应与冲击压缩特性

 基于量子化学从头计算方法,计算了处于高密度条件下氮分子之间的两体排斥势和三体关联势,并采用球面平均近似求得了它们各自的高温平均势能值,得到了包括三体关联效应的等效两体势函数与分子间距的解析关系。借助分子流体微扰变分理论,并考虑van der Waals长程作用,分别采用总平均两体势和总等效两体势,计算了液氮的冲击压缩曲线。计算结果表明,在不需要考虑分子离解的压缩范围内（1～35 GPa）,三体关联效应的贡献使分子间总等效两体势函数比单独两个分子间总平均作用势函数明显软化,与实验Hugoniot数据结果一致。     

四体相互作用对固氦压缩特性的贡献

运用从头计算自洽场方法和原子团簇理论计算了高压下固氦原子间的四体势分量.计算结果表明四体分量对结合能的贡献为正；随着压缩度增大四体势的贡献比例变大.采用两体、三体、四体势和Aziz吸引势计算固氦零温状态方程并与实验测量相比较，结果表明两体势对固氦的压缩性贡献了过多的正效应，加入三体分量的修正，仅在低于10GPa时理论值与实验值相符很好，但考虑了四体势修正后能将理论值与实验值符合程度提高到 27GPa. 关键词： 状态方程 固氦 四体势 从头计算     

双原子分子激发态势能的自旋相关局域Hartree-Fock密度泛函理论方法

基于自旋相关局域Hartree-Fock (SLHF)势函数，本文提出了一种计算双原子分子激发态势能的密度泛函理论(DFT)方法，并将该方法应用于和的激发态势能曲线的计算。在只考虑交换能的情况下，本文的DFT计算结果与文献中精确方法和Hartree-Fock (HF)方法的结果符合的非常好，说明采用SLHF势函数作为交换势的DFT方法是一个很好的计算激发态势能的方法。本文还计算和探讨了电子的关联势函数和关联能，发现传统的近似方法在较大核间距的情况下大大低估了电子的关联能.     

平面团簇稳定结构的蒙特卡罗树搜索

以平面团簇为例提出了一种结合结构识别和蒙特卡罗树技术搜索稳定结构的新方法.体系原子之间的相互作用由两类模型势能函数来描述:Lennard-Jones二体势函数与基于Lennard-Jones势的三体势函数.考虑可能的三角晶格碎片作为候选结构,引入编号策略对结构进行快速识别,并运用蒙特卡罗树搜索研究稳定结构随着原子数增大的演化过程;对于能量较低的候选结构,进一步采取局域优化来获得对应体系的稳定结构.计算表明,Lennard-Jones二体势函数对应的三角晶格团簇更稳定;在特定的参数下,三体势函数对应的六角晶格团簇更稳定.结合结构识别和蒙特卡罗树搜索可以对候选结构空间进行高效扫描,在较短时间内更容易搜索到稳定的团簇结构,并可以与第一原理计算结合实现材料的结构预测.     

Rhn、Con(n=3～56)团簇的结构特性研究

采用半经验的Gupta多体势结合遗传算法对Rhn、Con(n=3~56)团簇的基态结构特性进行了系统的研究.除在n=18~40尺寸范围内有少数团簇的构型不同,两种团簇具有相似的几何结构.在Rhn、Con(n=3~56)团簇的生长中,存在类Ih构型与类fcc构型之间的竞争,对于n≤24,两种团簇都从紧致密堆积结构过渡为类二十面体构型,Rh38及Co38为具有Oh对称性的类fcc构型,从n=39开始,铑团簇和钴团簇都呈现出明显的Ih生长模式.两种团簇的平均束缚能随原子数目的增加而增大,且在所研究的尺寸范围内铑团簇的平均结合能高于钴团簇.Rhn、Con(n=3~56)团簇具有相同的幻数序列:n=13,19,23,38,55.     

Ⅱ-Ⅵ族化合物原子间两体势的第一性原理研究

本文采用基于第一性原理的Hartree-Fork (HF)方法和密度泛函理论(DFT)计算了CdS、CdSe、CdTe、HgTe等Ⅱ-Ⅵ族化合物的两体相互作用势,并与实验数据进行了比较.分析了计算中基组对计算结果的影响,以及各种近似方法的优缺点.采用Lennard-Jones势、Born-Mayer势和Morse势函数分别对势能值进行拟合,结果表明采用Morse势拟合的势能曲线与计算结果符合最好,说明普遍应用于共价分子的Morse势也同样适用于对Ⅱ-Ⅵ族二聚体的两体势描述.     

相互作用势对激光辅助电子-氩原子散射的影响

在三种势模型下运用二级玻恩近似理论计算了电子与氩原子散射的微分散射截面. 结果显示这些势模型在激光辅助电子-氩原子散射系统中的运用非常成功. 另外,分析和讨论了静电势、交换势和极化势对散射截面的影响.     

用能量自洽法研究双原子分子离子XY+的势能函数

基于能量变分思想,建立了双原子分子离子XY+势能函数的新解析形式--ECMI势.该势函数中的离子库仑作用势包含了各级高阶修正,变分可调且收敛迅速.本文用这种新的解析势能函数和离子的能量自洽法(Energy-consistent-method for ion, ECMI)研究了双原子分子离子CO+基态X2Σ+态的势能函数.结果表明,新的解析势能函数ECMI势更加适合用来描述双原子分子离子XY+的全程势能行为.     

固氪物态方程的关联量子化学计算

运用多体展开理论和量子化学方法—–超分子单、双(三重)激发微扰处理耦合簇CCSD(T)方法,首次系统地计算了面心立方固氪在较宽(从晶格平衡位置到体积压缩率超过3倍)区间的两体、三体和四体相互作用对结合能和物态方程的贡献大小,包括Hartree-Fock自洽场项和范德瓦耳斯长程关联作用项;并与实验数据进行比较.结果表明,在考虑到两体、三体、四体相互作用能后,多体展开理论以及CCSD(T)方法对平衡位置结合能测量数据0—130 GPa整个研究区间的实验物态方程数据都做出令人满意的描述.     

Efimov physics in heteronuclear four-body systems

We study three- and four-body Efimov physics in a heteronuclear atomic system with three identical heavy bosonic atoms and one light atom. We show that exchange of the light atom between the heavy atoms leads to both three- and four-body features in the low-energy inelastic rate constants that trace to the Efimov effect. Further, the effective interaction generated by this exchange can provide an additional mechanism for control in ultracold experiments. Finally, we find that there is no true four-body Efimov effect-that is, no infinite number of four-body states in the absence of two- and three-body bound states-resolving a decades-long controversy.     

Bound-state calculations of the three-dimensional Yakubovsky equations with the inclusion of three-body forces

The four-body Yakubovsky equations in a three-dimensional approach with the inclusion of the three-body forces are proposed. The four-body bound state with two- and three-body interactions is formulated in the three-dimensional approach for identical particles as a function of vector Jacobi momenta, specifically, the magnitudes of the momenta and the angles between them. The modified three-dimensional Yakubovsky integral equations are successfully solved with the scalar two-meson exchange three-body force, where the Malfliet-Tjon-type two-body force is implemented. The three-body force effects on the energy eigenvalue and the four-body wave function, as well as accuracy of our numerical calculations are presented.     

压缩氦原子团簇Hen（n＝3，4，5）排斥势及其多体展开分量计算

 采用Lowdin方法计算了处于压缩状态的氦原子团簇He_n（n＝3，4，5）的排斥势及其多体展开分量。发现随原子数目或压缩度增大，多体展开式的收敛性变差。多体展开式中两体势分量、四体势分量为正值，三体势分量和五体势分量为负值。因此，在进行近似处理时，两体近似法计算的排斥势偏高。经三体修正后的排斥势必然偏低。该计算结果与最新的实验结果定性符合。发现四体势和五体势分量对高密度氦状态方程仍然具有重要贡献。     

Energy levels calculation for Er3+: LiYF4

The complete configuration 4f¹¹ of Er³⁺ was diagonalized with a Hamiltonian containing electrostatic, two-body and three-body configuration interaction, spin-orbit and crystal field parameters of LiYP₄. A set of 18 parameters was adjusted to fit the experimental energy levels as given by Petrov and Tkachuk.     

Analysis of semi-empirical interatomic potentials appropriate for simulation of crystalline and liquid Al and Cu

We investigate the application of embedded atom method (EAM) interatomic potentials in the study of crystallization kinetics from deeply undercooled melts, focusing on the fcc metals Al and Cu. For this application, it is important that the EAM potential accurately reproduces melting properties and liquid structure, in addition to the crystalline properties most commonly fit in its development. To test the accuracy of previously published EAM potentials and to guide the development of new potential in this work, first-principles calculations have been performed and new experimental measurements of the Al and Cu liquid structure factors have been undertaken by X-ray diffraction. We demonstrate that the previously published EAM potentials predict a liquid structure that is too strongly ordered relative to measured diffraction data. We develop new EAM potentials for Al and Cu to improve the agreement with the first-principles and measured liquid diffraction data. Furthermore, we calculate liquid-phase diffusivities and find that this quantity correlates well with the liquid structure. Finally, we perform molecular dynamics simulations of crystal nucleation from the melt during quenching at constant cooling rate. We find that EAM potentials, which predict the same zero-temperature crystal properties but different liquid structures, can lead to quite different crystallization kinetics. More interestingly, we find that two potentials predicting very similar equilibrium solid and liquid properties can still produce very different crystallization kinetics under far-from-equilibrium conditions characteristic of the rapid quenching simulations employed here.     

FCC金属空位的MAEAM理论研究

应用涉及更远邻原子的改进分析型嵌入原子方法（MAEAM）计算了面心立方（fcc）金属（Ag,Al,Au,Cu,Ir,Ni,Pd,Pt,Rh）的空位性能。在MAEAM计算中,考虑了远邻原子相互作用和单空位迁移能,对两体势进行了坚挺处理,并采用新的截尾函数和加强光滑连接条件对两体势作了截尾处理。同时为了更好的符合面心立方晶体的结合能、弹性常数和平衡条件,调整了多体势的模型常数。未弛豫空位性能计算中考虑了两体势的截尾距离和电子密度分布函数的截尾距离之间近邻原子的作用以及双空位迁移途径周围的原子非对称分布。结果与其它方法计算结果基本一致,但更加接近实验值。对双空位迁移能的计算结果有利地说明了fcc金属双空位5种迁移途径的扩散机制。     

Dreikörperkräfte und die Stabilität einfacher Kristallgitter

Observed stability of crystal structures for rare-gas atoms, alkali halides and for those II–VI and III–V compounds whose ions are isoelectronic with rare-gas atoms is correlated with atomic and ionic interactions in dense media. On the basis of central forces essential discrepancies between theory and experiment occur. Further analysis indicates that strong, simultaneous interactions of exchange type between three atoms or three ions play an important role in determining the stable crystal structure. An evaluation of such three-body interactions is carried out in first and second orders of perturbation theory, using effective-electron wave functions of Gaussian form. It is shown that the theory accounts for all observed stability relations on a quantitative basis. The stability of the face-centered cubic configuration for solids of heavy rare gases is explained. Also, the occurrence of the cesium chloride structure for some of the heavy alkali halides and the magnitude of observed transition pressures are reproduced by the theory. In addition, it is shown that three-ion interactions are responsable for the occurrence of the sphalerite (zincblende) and wurtzite configurations with II–VI and III–V compounds, without covalent bonding between the ions being involved. General laws are given which govern the relative stability of the sodium chloride, cesium chloride, sphalerite and wurtzite configurations for these types of compounds.