气液态分子运动的Hurst指数及相图研究

本文运用分子动力学模拟方法,模拟了不同温度条件下饱和气体和液体氩分子的运动。对气液状态下分子运动在速度空间的演化相图进行了比较。根据分形理论,对实际分子的分数布朗运动特性进行了讨论。应用本文提出的约化规模法,获得了不同温度条件下分子分数布朗运动的Hurst指数和轨迹分维数。饱和液相分子运动的Hurst指数并不随温度单调上升,存在一个极小值。     

Melting of cubic boron nitride at extreme pressures

Due to its large pressure range of stability and inert nature, cubic boron nitride has been proposed as a potential pressure standard for high pressure experiments. It is extremely refractive upon compression, although its melting temperature is not known beyond 10 GPa. We apply first-principles molecular dynamics to evaluate the thermodynamics of zincblende structured (cubic) and liquid boron nitride at extreme temperatures and pressures, and compute the melting curve up to 1 TPa by integration of the Clapeyron equation. The resulting equations of state reveal that liquid boron nitride becomes denser than the solid phase at pressures of around 0.5 TPa. This is expressed as a turnover in the melting curve, which reaches a maximum at 510 GPa and 6550 ± 700 K. The origin of this density crossover is explained in terms of the underlying liquid structure, which diverges from that of the zincblende structured solid as the phases are compressed.     

Phase transitions in simple clusters

Formation of the liquid state of clusters with pairwise interactions between atoms is examined within the framework of the void model, in which configurational excitation of atoms results from formation of voids. Void parameters are found from computer simulation by molecular dynamics methods for Lennard-Jones clusters. From that standpoint, phase transitions are analyzed in terms of two aggregate states. This information allows us to divide the entropy jump during a solid-liquid phase transition into two parts: one corresponds to configurational excitation at zero temperature and the other arises from thermal vibrations of atoms. The latter part contributes approximately 40% for Lennard-Jones clusters consisting of 13 and 55 atoms, increasing to 56% for bulk inert gases. These magnitudes explain the validity of melting criteria based on thermal motion of atoms, even though the distinctive mechanism of this phase transition results from configurational excitations. It is shown that the void concept allows analyzing various aspects of the liquid state of clusters including the existence of a limiting freezing temperature below which no metastable liquid state exists, as well as the existence and properties of glassy states that may exist below the freezing limit.     

Pressure effects on diffusion in liquid ammonia : A simulation study using a combination of isobaric-isothermal and microcanonical molecular dynamics

The effects of pressure on translational and rotational diffusion in liquid ammonia are investigated by means of molecular dynamics simulations. Calculations are done at two different temperatures and at many different pressures by using a two-part protocol involving molecular dynamics in isobaric-isothermal ensemble in the first part and in microcanonical ensemble in the second part. Our results are analyzed in terms of pressure-induced changes in structural properties such as packing and hydrogen bond properties. Also, the present results of liquid ammonia are compared with corresponding results for other hydrogen bonded liquids that were reported in recent years.      

Size-dependent surface tension of a cylindrical nanobubble in liquid Ar

In view of the continued disputes on the fundamental question of whether the surface tension of a vapour bubble in liquid argon increases,or decreases,or remains unchanged with the increase of curvature radius,a cylindrical vapour bubble of argon is studied by molecular dynamics simulation in this paper instead of spherical vapour bubble so as to reduce the statistical error.So far,the surface tension of the cylindrical vapour bubble has not been studied by molecular dynamics simulation in the literature.Our results show that the surface tension decreases with radius increasing.By fitting the Tolman equation with our data,the Tolman length δ = -0.6225 sigma is given under cut-off radius 2.5σ,where σ = 0.3405 nm is the diameter of an argon atom.The Tolman length of Ar being negative is affirmed and the Tolman length of Ar being approximately zero given in the literature is negated,and it is pointed out that this error is attributed to the application of the inapplicable empirical equation of state and the neglect of the difference between surface tension and an equimolar surface.     

Molecular dynamics studies of thermal conductivity time correlation functions

The Green–Kubo time correlation function for the thermal conductivity in liquid argon is studied for a thermodynamic state close to the triple point by standard molecular dynamics simulations. The collective heat flux vector has been separated into contributions originated at the kinetic energy, the intermolecular potential and the pair virial function. Furthermore, the Green–Kubo time correlation functions have been broken down into partial n-body terms (n=1,2,3,4). The most important contribution to the thermal conductivity is represented by the auto correlation of the virial term. In contrast to other collective phenomena described by time correlation functions involving n-body terms, the partial Green–Kubo time correlation functions for the thermal conductivity are not affected by exponential long-time tails.     

Computer simulations of structure, dynamics, and phase behavior of colloidal fluids in confined geometry and under shear

Using computer simulations, colloidal systems in different external fields are investigated. Colloid-polymer mixtures, described in terms of the Asakura-Oosawa (AO) model, are considered under strong confinement. Both in cylindrical and spherical confinement, the demixing transition of the three-dimensional AO model is rounded and, using Monte Carlo simulations, we analyze in detail the consequences of this rounding (occurrence of multi-domain states in cylindrical geometry, non-equivalence of conjugate ensembles due to different finite-size corrections in spherical geometry etc.). For the case of the AO model confined between two parallel walls, spinodal decomposition is studied using a combination of molecular dynamics simulation and the multiparticle collision dynamics method. This allows us to investigate the influence of hydrodynamic interactions on the domain growth during spinodal decomposition. For a binary glass-forming Yukawa mixture, non-linear active micro-rheology is considered, i.e. a single particle is pulled through a deeply supercooled liquid. The diffusion dynamics of the pulled particle is analyzed in terms of the van Hove correlation function. Finally, the Yukawa mixture in the glass state, confined between walls, is studied under the imposition of a uniform shear stress. Below and around the yield stress, persistent creep in the form of shear-banded structures is observed.     

EPR Spin Probe Study of New Micellar Systems

Molecular dynamics, structure, and phase state of two new micellar systems were investigated using spin-probe electron paramagnetic resonance spectroscopy. While the local mobility of the new cationic long-chain detergents varies in micelles in a minor way when the length of a chain increases from 16 to 22 carbon atoms, the order parameter increases noticeably. The latter is caused by gain of hydrophobic interactions. It is worthwhile to note that the incorporation of the two hydroxyl groups into the polar head group of the C22 detergent influences particularly on the molecular dynamics and phase state of an aqueous solution of the detergent. Furthermore, not only the local mobility decreases and ordering factor increases, but also the phase state of the system changes, being transited from the solid to the micellar (liquid) state. Addition of the KCl salt in an aqueous solution of the long-chain detergents results in a decrease of local mobility and increase of ordering factor. The phase transitions are found to be caused by the addition of the salt. The cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium methyl sulfate was shown to decrease the critical concentration of micelle formation of the anionic detergent sodium dodecyl sulfate (SDS). Most likely this is because, being the counterion, the cationic monomer forms a dense layer around the SDS micelle. Binding of SDS micelle with the monomer strongly reduces the local mobility of the detergent.     

基于混合液晶分子动力学模拟比较液晶分子旋转黏度大小

提高液晶波前校正器的响应速度是增加液晶自适应光学系统校正带宽的关键, 而研究设计低旋转黏度的液晶分子是提高液晶波前校正器响应速度的根本方法. 利用原子水平上的分子动力学方法获得了目标分子的液相、向列相以及近晶相, 给出了理论计算液晶分子序参数以及旋转黏度的方法. 与此同时, 结合实验方法, 提出利用混合液晶分子动力学模拟来比较液晶分子旋转黏度的大小, 通过多次模拟、多起始点数据处理最大限度消除了因边界尺寸效应带来的数据波动, 最后给出了两种高性能液晶分子的具体比较结果. 这种分子动力学模拟方法能够探查分子结构细微差别对液晶相态以及旋转黏度的影响, 为设计低旋转黏度的液晶分子提供了理论支持, 必将为快速响应液晶材料的设计提供帮助.     

First-principles molecular dynamics simulations of the structure of germanium dioxide under pressures

We have carried out first-principles molecular dynamics simulations of glass and liquid germanium dioxide (GeO₂) over a wide range of pressure. Our results show that in the glass GeO₂ system nearly all Ge–O coordination environments are fourfold at low compression, whereas at high compression five- and sixfold coordination types coexist. In the liquid GeO₂ system although most Ge–O coordination environments are fourfold, some threefold coordination types exist at low compression. Pentahedral units also exist in the liquid state while less than that in the glass state. At high compression, pentahedral units disappear and GeO₆ octahedron is dominant in the liquid state going with some sevenfold coordination.     

Acetonitrile revisited: a molecular dynamics study of the liquid phase

The subject of this report is the calibration of a model of the liquid state of acetonitrile (methyl cyanide). The model describes the liquid in terms of molecular mechanics with each molecule of the liquid treated as a rigid body that is composed of three interaction sites, between which Coulomb and dispersion interactions are computed. A brief overview of the literature on such models is given and a set of parameters for the model is presented. The representation of liquid acetonitrile produced by the parameters is compared to that produced by several other parameter sets available in the literature. It is concluded that, of the parameter sets for the three-site molecular mechanics model that currently are available, under the simulation conditions used, the one presented produces the most rounded representation of the properties of liquid acetonitrile.     

Size-dependent surface tension of cylindrical nano-bubble in liquid Ar

In view of the continued disputes on the fundamental question whether the surface tension of vapour bubble in liquid argon increases, or decreases, or remains unchanged with the increase of curvature radius, the cylindrical vapour bubble of argon is studied by molecular dynamics simulation in this paper instead of spherical vapour bubble so as to reduce the statistical error. So far the surface tension of the cylindrical vapour bubble has not been studied by molecular dynamics simulation in the literature. Our results show that the surface tension decreases with radius increasing. By fitting Tolman equation with our data, the Tolman length δ =-0.6225 sigma is given under cut-off radius 2.5σ, where σ =0.3405 nm is the diameter of argon atom. The Tolman length of Ar being negative is affirmed and the Tolman length of Ar being approximately zero given in the literature is negated, and it is pointed that this error is attributed to the application of the inapplicable empirical equation of state and the neglect of the difference between surface of tension and equimolar surface.     

Quadratic-nonlinear Landau-Zener transition for association of an atomic Bose-Einstein condensate with inter-particle elastic interactions included

We study the strong coupling limit of a quadratic-nonlinear Landau-Zener problem for coherent photo- and magneto-association of cold atoms taking into account the atom-atom, atom-molecule, and molecule-molecule elastic scattering. Using an exact third-order nonlinear differential equation for the molecular state probability, we develop a variational approach which enables us to construct a highly accurate and simple analytic approximation describing the time dynamics of the coupled atom-molecule system. We show that the approximation describing time evolution of the molecular state probability can be written as a sum of two distinct terms; the first one, being a solution to a limit first-order nonlinear equation, effectively describes the process of the molecule formation while the second one, being a scaled solution to the linear Landau-Zener problem (but now with negative effective Landau-Zener parameter as long as the strong coupling regime is considered), corresponds to the remaining oscillations which come up when the process of molecule formation is over.     

Simulation study of solvent shifts of vibrational overtone spectra

Buckingham's theory of the solvent shift of vibrational spectral frequencies predicts that the shift of the v = 0 → n overtone transition is n times the shift of the fundamental v = 0 → 1. We test this prediction by molecular dynamics simulations using existing intermolecular potential models for liquid N₂ and dilute N₂ in liquid Ar, at standard state conditions. We extend Buckingham's theory by including additional intramolecular potential and perturbation terms which lead to solvent-induced anharmonicity, i.e. O(n ²) terms in the solvent shift. The simulations show that Buckingham's prediction is not accurate for N₂ at standard liquid state conditions. We find that at these conditions there is a significant positive O(n ²) contribution to the solvent shifts and that for n ～ 20 the shifts change sign from red to blue. Simulation results and indirect evidence from shock wave experiments with liquid N₂ show that Buckingham's prediction is more accurate for high-pressure high-temperature conditions, where the shifts are blue and only slightly nonlinear in n.     

Nuclear quantum effects in water

A path-integral Car-Parrinello molecular dynamics simulation of liquid water and ice is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first-principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed open path-integral molecular dynamics methodology. It is shown that these results are in good agreement with experimental data.     

The behavior of crowdions and their complexes in weakly stable states of materials

Using the method of molecular dynamics, both single crowdions and their complexes are investigated in an fccmaterial being in a low-stability state with respect to an external thermal force. The system’s state with single crowdions is shown to be weakly stable, and their complexes therefore present a more favorable interstitial position. It is established that even under moderate external action (thermal activation or propagation of a standing wave) single crowdions form a dumbbell defect. In a low-stability state that is developed in the vicinity of structural-phase transformations, it is the crowdion complexes which represent more stable configurations. In addition, the bulk crowdion complex appears to be most favorable, being virtually unaffected by the phonon friction.     

Molecular conformation and liquid structure of 2-propanol through neutron diffraction

The neutron diffraction data analysis of deuterated liquid 2-propanol at room temperature to define its molecular conformation is presented. 2-Propanol being a large molecule with twelve atomic sites, the conformation analysis is tricky and an improved method of data analysis is given. The intermolecular structural correlations, i.e., hydrogen-bonded liquid structure, can be modelled accurately to extract the nature of the average hydrogen-bonded molecular association in liquid state at room temperature. Like other alcohols these are mostly hexamer ring chain (HRC) clusters. The cluster analysis of recent X-ray data available in the literature also support the same liquid structure.     

Dynamics of capillary drying in water

We use atomistic simulations to address the question when capillary evaporation of water confined in a hydrocarbonlike slit is kinetically viable. Activation barriers and absolute rates of evaporation are estimated using open ensemble Monte Carlo-umbrella sampling and molecular dynamics simulations. At ambient conditions, the evaporation rate in a water film four molecular diameters thick is found to be of the order 10(5)(nm(2) s)(-1), meaning that water readily evaporates. Films more than a few nanometers thick will persist in a metastable liquid state. Dissolved atmospheric gas molecules do not significantly decrease the activation barrier.