Continuum Model for Electronic Polarization Based on a Novel Dielectric Response Function

基于连续介质模型推导了一个普适的描述电介质介电弛豫过程的响应函数. 该介电响应函数依赖于电介质的介电谱. 基于该函数推导得到了以前特殊情况下用于描述溶剂弛豫的响应函数一致的表达式.结合三种典型极性溶剂,水、甲醇和乙腈的介电谱,研究了三种溶剂在外加电场线性变化时的电子极化过程. 结果表明,溶剂的电子极化伴随着电子跃迁同步发生,没有时滞.     

金属介电函数的正确运用——以金属纳米球诱导的二能级系统的基态能级移动为例

金属对电磁场的响应包括自由和束缚电荷两部分的贡献,大学物理教学中通常分别用Drude模型和Lorentz模型描述,然而,以上模型所得的介电函数只在一段较窄的频率范围内能与实验数据吻合.因此,在实际科研工作中,当物理量与较宽频率范围内的电磁场响应有关时,原则上介电函数应当采用实验测量值而不能采用模型化的结果.本文以金属纳米球诱导的二能级系统的基态能级移动为例,分别展示采用模型化的介电函数和实验介电函数时,能级移动计算结果的差异.     

负性介电各向异性向列相液晶中空间光孤子的理论研究

对非局域非线性介质向列相液晶中介电各向异性为负时的情况进行了研究.理论研究表明,负性介电各向异性的向列相液晶具有负的非线性系数.文中给出了其空间非局域响应特征宽度和非线性系数的表达式,并求出了其非局域响应函数;其次,用数值计算的方法给出了其空间孤子的传输结果.最后,研究了光束功率和偏置电压的改变对负性介电各向异性向列相液晶中光束传输的影响,发现偏置电压的改变会导致光束在负性介电各向异性液晶中形成孤子所需的临界功率发生改变.     

Schottky势垒高度理论计算中的平均键能方法

以平均键能Em作为参考能级，计算了１０种不同半导体的Schottky接触势垒高度，计算值与实验值符合较好-计算值与实验值的符合程度与Tersoff的电中性能级EB方法相当，优于Harrison和Cardona等人采用sp3平均杂化能εh和介电函数隙中能级ED的计算结果- 关键词： 势垒高度 平均键能方法 费米能级     

立方晶相HfO2电子结构与光学性质的第一性原理计算

利用密度泛函理论框架下的第一性原理平面波超软赝势方法计算了立方晶相二氧化铪(c-HfO2)的电子结构,得到了c-HfO2的总念密度、分波态密度和能带结构.经带隙校正后,计算了c-HfO2的光学线性响应函数随光子能最的变化关系,包括复介电函数,反射率、复折射率以及光学吸收系数,并从理论上给出了c-HfO2材料光学性 质与电子结构的关系.经比较发现,对c-HfO2的电子结构和光学性质的计算结果与已有的实验数据和其它理论研究吻合得较好,从而为c-HfO2光电材料的设计与应用提供了理论依据.同时,计算结果也表明采用密度泛函理论的广义梯度近似来计算和预测c-HfO2材料的电子结构和光学性质是比较可靠的.     

二维随机介质中柱面波传播及其局域性的随机泛函分析

分析了二维各向同性均匀随机介质中柱面波的传播特性及局域化现象.用随机泛函理论,在频域内将随机介电起伏展开成柱坐标系下的Wiener积分式,将波场表示为内外行柱面波的线性和,求解二维Helmholtz波动方程,得到随机介电起伏对柱面波幅度与相位调制的解析表达.由柱面波能量的空间分布验证了波的局域化现象,并求解局域化长度.二维随机介质中平面波按柱面波展开的波转换方程与非随机介质中的情形有相似的表达,但具有随机介电起伏对幅度和相位的调制,并给出数值模拟结果.     

ZnO电子结构与光学性质的第一性原理计算

计算了ZnO电子结构和光学线性响应函数,从理论上给出了ZnO材料电子结构与光学性质的关系。所有计算都是基于密度泛函理论框架下的第一性原理平面波超软赝势方法。利用精确计算的能带结构和态密度分析了带间跃迁占主导地位的ZnO材料的介电函数、反射谱、反射率以及消光率,理论结果与实验符合甚佳,为ZnO光电材料的设计与应用提供了理论依据。同时,计算结果也为精确监测和控制ZnO材料的生长过程提供了可能性。     

利用磁光克尔效应计算材料的介电张量非对角元

利用磁控溅射的方法制备了Ag和Ag/Co双层薄膜样品.通过椭偏测量和磁光克尔测量,得到薄膜样品的有效光学常量和有效磁光参量,并通过数值分析得到介电张量的非对角元.实验结果表明:用通常公式计算和磁光测量与椭偏测量结合计算非磁样品Ag薄膜的光学常量的结果一致,验证了基于磁光测量方法计算介电张量非对角元的正确性.     

静压下Zn1-xCdxSe/ZnSe窄量子阱的激子和光跃迁

利用波恩公式近似建立了应变与介电常量的定量关系考虑应变对介电常量、有效质量、晶格常量(体积)等诸多物理量的影响,用变分法计算了静压下Zn_1-xCd_xSe/ZnSe窄量子阱中激子结合能和光跃迁能量随压力的变化理论计算结果与其他作者的实验和理论结果进行了比较和讨论.     

弛豫铁电体介电可调性的研究

无铅弛豫铁电体具有较好的介电可调性,在顺电相有较大的介电常数和极小的损耗,因较大的优值而被广泛地用于微波器件.根据现有的介电可调性理论,通过参量的适当修正,对介电可调性的表达式做了合理的探讨,结论适用于处理实验结果.比较发现,在电场作用下顺电相保持不变的近似得出的结论与实验结果差距较大,而转化为铁电相与实验结果完全吻合.考虑外加电场和自发极化对弹性吉布斯自由能的修正,导出了高电场对介电常数的修正关系,与实验结果相符.提出了介电可调度的概念与计算公式,能够定量表示掺杂对介电可调性的影响.     

Coupling hydrophobicity, dispersion, and electrostatics in continuum solvent models

An implicit solvent model is presented that couples hydrophobic, dispersion, and electrostatic solvation energies by minimizing the system Gibbs free energy with respect to the solvent volume exclusion function. The solvent accessible surface is the output of the theory. The method is illustrated with the solvation of simple solutes on different length scales and captures the sensitivity of hydration to the particular form of the solute-solvent interactions in agreement with recent computer simulations.     

Ion solvation dynamics in supercritical fluids

We present a theoretical study of ion solvation dynamics in a supercritical solvent. Molecular dynamics simulations show a significant difference between equilibrium and nonequilibrium solvent response functions, especially pronounced at medium and low solvent densities. We propose a simple analytical theory for the nonequilibrium solvation function based on the generalized nonlinear Smoluchowski-Vlasov equation. The theory is shown to be in excellent agreement with simulation over a wide range of supercritical solvent densities.     

UV-spectral changes for some azo compounds in the presence of different solvents

The electronic absorption spectra of four azo dyes with different substituents (such as Cl, I, OH) are determined at room temperature in twenty-one solvents with different polarities. The electronic transitions of azo dyes are interpreted. Linear solvation energy relationships have been investigated for solvatochromic behaviors and solute-solvent interactions of azo dyes. Linear solvation energy relationships were performed by multiple linear regression analysis using dielectric function, refractive index function and Kamlet-Taft parameters. We have observed that the hydrogen bonding acceptor ability and the induction-dispersive forces of solvent molecules have caused the bathochromic shift in absorption maxima of azo dyes.     

On the signs of fluorine spin coupling constants

From an exact expression for the free energy of a non-uniform fluid mixture a closure approximation for the inhomogeneous direct correlation functions is used to develop a theory of solvation forces in charged fluids based upon non-linear equations for the equilibrium ion number densities. In the limit of point ions, the expressions obtained reduce to those of the Poisson-Boltzmann theory of electrolytes. The numerical results obtained for a restricted primitive model electrolyte are compared with those of earlier work based on linear response theory and Poisson-Boltzmann theory with a simple Stern layer modification. At low electrolyte concentrations the agreement between all three theories is good. But at high electrolyte concentrations the Poisson-Boltzmann theory with a simple Stern layer correction fails to display the oscillations in the solvation force which characterize both the linear and non-linear theories.     

Differential geometry based solvation model I: Eulerian formulation

This paper presents a differential geometry based model for the analysis and computation of the equilibrium property of solvation. Differential geometry theory of surfaces is utilized to define and construct smooth interfaces with good stability and differentiability for use in characterizing the solvent-solute boundaries and in generating continuous dielectric functions across the computational domain. A total free energy functional is constructed to couple polar and nonpolar contributions to the salvation process. Geometric measure theory is employed to rigorously convert a Lagrangian formulation of the surface energy into an Eulerian formulation so as to bring all energy terms into an equal footing. By minimizing the total free energy functional, we derive coupled generalized Poisson-Boltzmann equation (GPBE) and generalized geometric flow equation (GGFE) for the electrostatic potential and the construction of realistic solvent-solute boundaries, respectively. By solving the coupled GPBE and GGFE, we obtain the electrostatic potential, the solvent-solute boundary profile, and the smooth dielectric function, and thereby improve the accuracy and stability of implicit solvation calculations. We also design efficient second order numerical schemes for the solution of the GPBE and GGFE. Matrix resulted from the discretization of the GPBE is accelerated with appropriate preconditioners. An alternative direct implicit (ADI) scheme is designed to improve the stability of solving the GGFE. Two iterative approaches are designed to solve the coupled system of nonlinear partial differential equations. Extensive numerical experiments are designed to validate the present theoretical model, test computational methods, and optimize numerical algorithms. Example solvation analysis of both small compounds and proteins are carried out to further demonstrate the accuracy, stability, efficiency and robustness of the present new model and numerical approaches. Comparison is given to both experimental and theoretical results in the literature.     

Free energies of supercritical solvation from molecular dynamics simulation and integral equation studies

We present here molecular dynamics (MD) simulation and integral equation (IE) studies on free energies of solvation of a non-polar solute in a dilute supercritical solvent to estimate the contribution of inhomogeneities in solvent density to the free energy of solvation. The solvation of a Xe-like solute in an Ne-like solvent as well as that of naphthalene in CO₂ have been investigated. At state points in the compressible region in the neighborhood of the solvent critical point, we have utilized the IE estimates of free energies to model the ideal situation where local density inhomogeneities would be absent. The difference between the free energies in the presence (as derived from MD simulation) and in the absence (from IE) of local density inhomogeneities was studied as a function of density along an isotherm close to the critical point. Although for low density supercritical solvents, a marked difference is observed, a study of the density dependence of this difference across the critical density does not directly reveal any signature of local density enhancement on the thermodynamics of solvation.     

General solvation motifs of a charged linear macroion in an aqueous droplet

ABSTRACT

We explore the solvation patterns of a charged rigid and semi-rigid linear macroion in an aqueous droplet. The solvation patterns are summarised in an empirical ‘phase diagram’ on the parameter space defined by the length of the macroion and its charge density. In the study, we employ molecular dynamics and atomistic modelling. The macroion is represented by a positively charged carbon nanotube. Linear macroion-solvent interactions in droplets are distinct from those of spherical ions because of the interplay among several factors such as the tendency of the solvent to form spherical droplets in order to minimise the surface energy, the constraint on the charge of a spherical droplet imposed by the Rayleigh limit, the solvation energy of the macroion and its length. The combination of all these factors may lead to a variety of solvent distributions along the rigid rod such as asymmetric solvation of the linear macroion, formation of spiky ‘star’-like distribution of solvent, partial wetting of the rod by a droplet. The study provides insight into the solvation of macroions in droplets with applications in electrosprayed macroions and atmospheric aerosols. We also propose a possible path of generating a sequence of nanoparticles of different shapes (spheres, multi-point stars) along a linear macromolecule by exploiting the various solvation patterns.     

Projections of quantum observables onto classical degrees of freedom in mixed quantum-classical simulations: understanding linear response failure for the photoexcited hydrated electron

We present a general analytic method for understanding how specific motions of a classical bath influence the dynamics of quantum-mechanical observables in mixed quantum-classical molecular dynamics simulations. We apply our method and develop expressions for the special case of quantum solvation, allowing us to examine how specific classical solvent motions couple to the equilibrium energy fluctuations and nonequilibrium energy relaxation of a quantum-mechanical solute. As a first application of our formalism, we investigate the motions of classical water underlying the equilibrium and nonequilibrium excited-state solvent response functions of the hydrated electron; the results allow us to explain why the linear response approximation fails for this system.     

Theoretical investigation of imidazolium based ionic liquid/alcohol mixture: a molecular dynamic simulation

In this work, molecular dynamic simulation of the mixture of imidazolium based ionic liquids with alcohols is implemented in order to investigate mixing excess properties and some structural and physical properties of the mixture. Excess volumes and enthalpies are evaluated for 11 different mole fractions of ionic liquids at each 0.1, in the range of 0 to 1. Radial distribution function, cohesive energy density, potential of mean force, solvation energy, and diffusion coefficient are reported and analysed. The effects of the cationic alkyl chain length, in comparison with changes of the anions, on these properties are reported. Results reveal that the methanol molecule participates with its hydrophilic characteristics in the mixing process and tends to aggregate around anion part of the ionic liquids, especially in the case of Cl.     

Nanoscale molecular superfluidity of hydrogen

We present a microscopic quantum theoretical analysis of the nanoscale superfluid properties of solvating clusters of para-H2 around the linear OCS molecule. Path-integral calculations with N=17 para-H2 molecules, constituting a full solvation shell, show the appearance of a significant superfluid response to rotation around the molecular axis at T=0.15 K. This low-temperature superfluid response is highly anisotropic and drops sharply as the temperature increases to T approximately 0.3 K. These calculations provide definitive theoretical evidence that an anisotropic superfluid state exists for molecular hydrogen in this microscopic solvation layer.