一个新的桥泛函及其在非均一流体密度泛函理论中的应用

基于对OZ 方程的渐近行为与Taylor级数展开的分析，提出了一个新的桥泛函，桥泛函被表达为间接相关函数的函数，Taylor级数展开的重整化导致了一个可调参数，通过将所提出的桥泛函与一个最近提出的密度泛函理论方法学，以及单个硬墙的sum 规则结合，可以确定可调参数.所提出的桥泛函能预言如下非均一流体的密度分布：硬球流体接近一个硬墙与在球形空隙内，Lennard-Jones 流体与缔合硬球流体在两个硬墙之内.理论预言与文献所报导的模拟数据符合很好.     

AB2型聚合物流体的表面结构性质

在密度泛函理论(DFT)框架下, 应用改进的基本度量理论(MFMT)表达硬球作用对自由能泛函的贡献, 根据统计力学理论结合加权密度近似(WDA)表达聚合作用对自由能泛函的贡献, 建立了描述AB2型聚合物流体的化学势, 得到了聚合物流体在硬球颗粒表面的密度分布表达式, 计算了聚合物流体在硬球颗粒表面附近的密度分布, 并探讨了体积分数、聚合程度和硬球颗粒尺度对体系密度分布的影响. 此外, 通过体系密度分布, 进一步分析了体积分数、聚合程度和硬球颗粒尺度与剩余吸附的关系.     

平行硬壁间荷电硬球流体的密度泛函理论研究

基于经典流体的密度泛函理论并结合改进的基本度量理论, 研究了受限于平行硬壁间的荷电硬球流体的平衡密度分布. 通过对比有、无外电场时分子数密度分布的相应变化情况, 分析了荷电硬球分子间的库仑排斥势能与场致势能间的竞争作用, 研究了电场对于受限荷电硬球流体系统聚集态结构的影响.     

氢气在碳纳米管内吸附的密度泛函理论研究

本文应用密度泛函理论研究纯流体氢气在单壁碳纳米管内吸附过程,采用硬球状态方程改进的基本测量理论表征硬球的斥力作用,离子吸引项的贡献则用微扰理论描述.在温度为300k,氢气本体对比密度范围为0.2～0.7条件下,计算了三种不同尺寸的碳纳米管氢气吸附的密度分布,其密度泛函计算结果与计算机分子模拟数据完全一致.     

平行板间超支化聚合物流体的密度分布和溶剂化力

在密度泛函理论(DFT)框架下,应用改进的基本度量理论(MFMT)和统计力学理论结合加权密度近似(WDA)分别表示硬球作用和聚合作用对自由能泛函的贡献,建立了描述ABg型超支化聚合物流体的化学势,得到了聚合物流体在平行板间的密度分布表达式,计算了聚合物流体在两平行板间的密度分布,并探讨了体积分数、反应程度和单体中B类官能团数对体系密度分布的影响.此外,通过体系密度分布,进一步分析了反应程度、板间宽度与溶剂化力的关系.     

基于密度泛函理论研究二元排斥Yukawa流体的表面结构性质

基于自由能密度泛函理论(DFT)考察了二元排斥Yukawa (HCRY)流体在不同外场下的密度分布. 基于微扰理论, 体系的Helmholtz自由能泛函采用硬球排斥部分和长程色散部分贡献之和, 其中Kierlik和Rosinberg的加权密度近似(WDA)被用来计算硬球排斥部分, 而色散部分采用平均场理论(MFT)进行描述. 为了验证DFT计算结果的合理性, 研究中采用巨正则Monte Carlo(GCMC)模拟计算了在不同主体相密度、硬核直径和位能参数比的条件下二元HCRY混合流体的密度分布. 结果表明, 该DFT计算结果与GCMC模拟值吻合良好.     

L-蛋氨酸对预腐蚀Q235碳钢缓蚀性能的分子动力学研究

采用失重法、密度泛函理论和分子动力学模拟研究了预腐蚀对L-蛋氨酸在60℃、1M的HCl溶液中对Q235碳钢的影响和缓蚀剂的缓蚀机理。结果表明:预腐蚀作用降低了L-蛋氨酸对Q235碳钢缓蚀作用,缓蚀率下降15%。密度泛函理论和分子动力学模拟结果表明,羧基上的氧原子带有明显的负电荷能够吸附到金属表面形成缓蚀剂膜,发挥缓蚀作用,为成膜缓蚀机理。L-蛋氨酸在没有预腐蚀的Q235碳钢表面吸附分子数为24时,缓蚀剂膜最致密。而L-蛋氨酸在预腐蚀后的Q235碳钢表面吸附时的吸附能显著降低,缓蚀剂在碳钢表面的分布密度也明显下降,这很好解释了预腐蚀作用降低了缓蚀剂缓蚀率的实验结果。本文的研究结果对缓蚀剂的研究和实际应用具有一定的指导意义。     

取代基对依布硒啉电子结构及光谱性质影响的理论研究

采用密度泛函理论B3LYP方法在6-311++G**水平对依布硒啉及其6个衍生物进行结构优化,并分析了取代基对分子几何构型、电荷分布、前线轨道能级分布的影响.在相同计算水平上采用含时密度泛函理论方法进行了电子光谱研究,讨论了取代基对电子光谱的影响.计算结果表明,标题化合物分子为非平面结构,苯并异硒唑酮环与苯环之间存在一...     

密度泛函理论及其数值方法新进展

综述了密度泛函理论及其数值方法的最新进展.密度泛函理论的发展以寻找合适 的交换相关近似为主线,从最初的局域密度近似、广义梯度近似到现在的非局域泛函、自相 互作用修正,多种泛函形式的相继出现使得密度泛函理论可以提供越来越精确的计算结果. 除了交换相关近似的发展,近年来密度泛函理论向含时理论、相对论等方面的扩展也很活跃 .另外,在密度泛函理论体系发展的同时,相应的数值计算方法的发展也非常迅速.从古老 的有限差分、有限元到新兴的小波分析都被用来实现密度泛函理论的数值计算.与此同时, 线性标度的密度泛函理论算法日趋成熟,使得通过密度泛函理论研究诸如生物大分子之类的 体系成为可能.随着密度泛函理论本身及其数值方法的发展,它的应用也越来越广泛,一些新的应用领域和研究方向不断涌现.     

规则星形高分子之间的相互作用势的自洽场研究

用自洽场理论方法计算了星形高分子之间的相互作用势, 并与标度理论的推测以及基于粒子的分子动力学模拟方法的结果进行了比较. 自洽场理论计算得到的星形高分子之间的二体相互作用势与标度理论推测的结果基本一致: 当两个星形高分子核心之间的距离小于其对应的等效软球的半径R时, 二体相互作用势U与星形高分子核心之间的距离d及官能度f之间的关系为U(d)∝-f 3/2 ln(d/R). 当星形高分子核心间距离较小, 相互作用势与星形高分子核心间距离的对数呈线性关系时, 三体相互作用为吸引作用, 但其强度约等于二体作用的10%. 因此, 以二体作用为基础的基于粒子的模拟方法是基本合理的.     

First-order mean spherical approximation for inhomogeneous fluids

The first-order mean-spherical approximation (FMSA) [Y. Tang, J. Chem. Phys., 118, 4140 (2003)] is extended to the studies of inhomogeneous fluids by combining with Rosenfeld's perturbative method [Y. Rosenfeld, J. Chem. Phys. 98, 8126 (1993)]. In the extension, the key input-direct correlation function of FMSA-is applied to constructing the free energy density functional. Preserving its high fidelity at the bulk limit, the FMSA shows satisfactory performance for Yukawa fluids near hard and attractive walls. The results are better than or comparable to several other theories reported before for the geometry. The FMSA is found, in particular, more satisfactory than the traditional mean-field theory for predicting density profiles around hard walls. The FMSA is also compared with the full MSA for inhomogeneous fluids, showing no appreciable differences. The inhomogeneous FMSA goes successfully through the self-consistency test for reproducing the radial distribution function of the bulk Yukawa fluid. As far as the computation is concerned, the FMSA can be executed much faster than any nonmean-field theories, and the speed is virtually identical to that of the mean-field theory.     

A molecular dynamics study of water between Lennard-Jones walls

An MD simulation of 216 ST2 water molecules between 12-6 Lennard-Jones walls has been performed which extend over 20 ps at an average temperature of 287 K. The oxygen atom density profile is reported the influence of the walls on the orientation of the water molecules on the self-diffusion coefficient have been investigated The results are compared with those from MC and MD simulations of similar systems.     

Solvation forces in ionic and neutral liquids: A density functional approach

The solvation forces between two planar charged surfaces in ionic solutions, corresponding to charged and neutral hard spheres representing the ions and the solvent, respectively, are studied here using a weighted density functional theory for inhomogeneous Coulomb systems developed by us recently. The hard sphere contributions to the one-particle correlation function are evaluated nonperturbatively using a position-dependent effective density, while the electrical contributions are obtained through a perturbative expansion around this weighted density. The calculated results on the solvation forces between two charged hard walls compare well with available simulation results for ionic systems. For a neutral system, the present results show good agreement with the experimentally observed oscillating forces for two mica surfaces in octamethylcyclotetrasiloxane. The present approach thus provides a direct route to the calculation of interaction energies between colloidal particles.     

Hard sphere fluids at a soft repulsive wall: a comparative study using Monte Carlo and density functional methods

Hard-sphere fluids confined between parallel plates at a distance D apart are studied for a wide range of packing fractions including also the onset of crystallization, applying Monte Carlo simulation techniques and density functional theory. The walls repel the hard spheres (of diameter σ) with a Weeks-Chandler-Andersen (WCA) potential V(WCA)(z) = 4ε[(σ(w)/z)(12) - (σ(w)/z)(6) + 1/4], with range σ(w) = σ/2. We vary the strength ε over a wide range and the case of simple hard walls is also treated for comparison. By the variation of ε one can change both the surface excess packing fraction and the wall-fluid (γ(wf)) and wall-crystal (γ(wc)) surface free energies. Several different methods to extract γ(wf) and γ(wc) from Monte Carlo (MC) simulations are implemented, and their accuracy and efficiency is comparatively discussed. The density functional theory (DFT) using fundamental measure functionals is found to be quantitatively accurate over a wide range of packing fractions; small deviations between DFT and MC near the fluid to crystal transition need to be studied further. Our results on density profiles near soft walls could be useful to interpret corresponding experiments with suitable colloidal dispersions.     

Comparative study between continuum and atomistic approaches of liquid flow through a finite length cylindrical nanopore

Steady state pressure driven flow of liquid argon through a finite length cylindrical nanopore was investigated numerically by classical Navier-Stokes (NS) hydrodynamic models and nonequilibrium molecular dynamics (MD) simulations. In both approaches, the nanopore was nominally 2.2 nm in diameter and 6 nm long. For the MD simulations, the intermolecular properties of the walls were specified independently from the liquid. Comparisons between the approaches were made in terms of the gross feature of total flow rate through the nanopore, as well as the more refined considerations of the spatial distributions of pressure, density, and velocity. The results showed that for the NS equations to predict the same trends in total flow rate with increasing pressure difference as the MD simulation, submodels for variations in density and viscosity with pressure are needed to be included. The classical NS boundary conditions quantitatively agreed with the flow rate predictions from MD simulations only under the condition of having a neutral-like solid-liquid interaction. Under these conditions, the NS and MD models also agreed well in streamwise distributions of pressure, density, and velocity, but not in the radial direction.     

Influence of atomistic physics on electro-osmotic flow: an analysis based on density functional theory

Molecular density profiles and charge distributions determined by density functional theory (DFT) are used in conjunction with the continuum Navier-Stokes equations to compute electro-osmotic flows in nanoscale channels. The ion species of the electrolyte are represented as centrally charged hard spheres, and the solvent is treated as a dense fluid of neutral hard spheres having a uniform dielectric constant. The model explicitly accounts for Lennard-Jones interactions among fluid and wall molecules, hard sphere repulsions, and short range electrical interactions, as well as long range Coulombic interactions. Only the last of these interactions is included in classical Poisson-Boltzmann (PB) modeling of the electric field. Although the proposed DFT approach is quite general, the sample calculations presented here are limited to symmetric monovalent electrolytes. For a prescribed surface charge, this DFT model predicts larger counterion concentrations near charged channel walls, relative to classical PB modeling, and hence smaller concentrations in the channel center. This shifting of counterions toward the walls reduces the effective thickness of the Debye layer and reduces electro-osmotic velocities as compared to classical PB modeling. Zeta potentials and fluid speeds computed by the DFT model are as much as two or three times smaller than corresponding PB results. This disparity generally increases with increasing electrolyte concentration, increasing surface charge density and decreasing channel width. The DFT results are found to be comparable to those obtained by molecular dynamics simulation, but require considerably less computing time.     

Structures and correlation functions of multicomponent and polydisperse hard-sphere mixtures from a density functional theory

The structures of nonuniform binary hard-sphere mixtures and the correlation functions of uniform ternary hard-sphere mixtures were studied using a modified fundamental-measure theory based on the weight functions of Rosenfeld [Rosenfeld, Phys. Rev. Lett. 63, 980 (1989)] and Boublik-Mansoori-Carnahan-Starling-Leland equation of state [Boublik, J. Chem. Phys. 53, 471 (1970); Mansoori et al., J. Chem. Phys. 54, 1523 (1971)]. The theoretical predictions agreed very well with the molecular simulations for the overall density profiles, the local compositions, and the radial distribution functions of uniform as well as inhomogeneous hard-sphere mixtures. The density functional theory was further extended to represent the structure of a polydisperse hard-sphere fluid near a hard wall. Excellent agreement was also achieved between theory and Monte Carlo simulations. The density functional theory predicted oscillatory size segregations near a hard wall for a polydisperse hard-sphere fluid of a uniform size distribution.     

基于基团贡献法和分子动力学预测聚间苯二甲酰对苯二胺的玻璃化转变温度

采用基团贡献法(GC)和分子动力学法(MD)模拟了聚间苯二甲酰间苯二胺纤维(MPDI)和聚对苯二甲酰对苯二胺(PPTA)的玻璃化转变温度, 并与实验值进行了对比. 结果表明, 使用基团贡献法和分子动力学法测得的MPDI和PPTA的玻璃化转变温度与实验值接近, 说明基团贡献法和分子动力学法可以用来预测芳香族聚酰胺的玻璃化转变温度. 在此基础上, 采用GC和MD预测了聚间苯二甲酰对苯二胺(PPIA)的玻璃化转变温度. 在MD模拟中, 对密度、 比体积、 回转半径和非键相互作用随温度的变化规律进行了分析. 结果表明, 自由体积理论能较好地解释PPIA的玻璃化转变现象, 其中非键相互作用随温度的变化是玻璃化转变的本质原因. PPIA的玻璃化转变温度介于MPDI和PPTA之间, 有望成为综合性能介于两者之间的另一种高性能聚酰胺.