A modified equation of state for Xe at high pressures by molecular dynamics simulation

The exact equation of state （EOS） for the fission gas Xe is necessary for the accurate prediction of the fission gas behavior in uranium dioxide nuclear fuel, However, the comparison with the experimental data indicates that the applicable pressure ranges of existing EOS for Xe published in the literature cannot cover the overpressure of the rim fission gas bubble at the typical UO2 fuel pellet rim structure. Based on the interatomic potential of Xe, the pressure-volume-temperature data are calculated by the molecular dynamics （MD） simulation. The results indicate that the data of MD simulation with Ross and McMahan＇s potential [M. Ross and A. K. McMahan 1980 Phys. Rev. B 21 1658] are in good agreement with the experimental data. A preferable EOS for Xe is proposed based on the MD simulation. The comparison with the MD simulation data shows that the proposed EOS can be applied at pressures up to 550 MPa and 3 GPa and temperatures 900 K and 1373 K respectively. The applicable pressure range of this EOS is wider than those of the other existing EOS for Xe published in the literature.

Equation of state for warm dense lithium： A first principles investigation

The quantum molecular dynamics based on the density functional theory has been adopted to simulate the equation of state for the shock compressed lithium. In contrary to some earlier experimental measurement and theoretical simulation,there is not any evidence of the ‘kink＇ in the Hugoniot curve in our accurate simulation. Throughout the shock compression process, only a simple solid-to-liquid melting behavior is demonstrated, instead of complicated solid–solid phase transitions. Moreover, the x-ray absorption near-edge spectroscopy has been predicted as a feasible way to diagnose the structural evolution of warm dense lithium in this density region.

Molecular Dynamics Simulations for Melting Temperatures of SrF2 and BaF2

The shell-model molecular dynamics method was applied to simulate the melting temper- atures of SrF2 and BaF2 at elevated temperatures and high pressures. The same method was used to calculate the equations of state for SrF2 and BaF2 over the pressure range of 0.1 MPa-3 GPa and 0.1 MPa-7 GPa. Compared with previous results for equations of state, the maximum errors are 0.3% and 2.2%, respectively. Considering the pre-melting in the fluorite-type crystals, we made the necessary corrections for the simulated melting temper- atures of SrF2 and BaF2. Consequently, the melting temperatures of SrF2 and BaF2 were obtained for high pressures. The melting temperatures of SrF2 and BaF2 that were obtained by the simulation are in good agreement with available experimental data.

Derivation of Martin-Hou Equation of State from Hard-particle Perturbation Theory

In this paper, the Martin-Hou equation of state is derived by using a power series representation of radial distribution function and an analytic representation of multi-section potential based on the Barker-Henderson hard-particle perturbation theory including high-order terms. In the derivation, a theoretical form of Martin-Hou equation was obtained. It had a similar form and the same capability to predict P-V-T properties as the Martin-Hou equation and no additional data were required for evaluating the constants. The characteristic constants of the theoretical expression have certain relationships with the molecular parameters.

Simulated Equations of State of MgSiO3 Perovskite

The equation of state of MgSiO3 perovskite under high pressure and high temperature is simulated using the molecular dynamics method. It was found that the molecular dynamics simulation is very successful in accurately reproducing the measured molar volumes of MgSiO3 perovskite over a wide range of temperatures and pressures. The simulated equation of state of MgSiO3 perovskite matched experimental data at up to 140GPa at 300K, as well as the fitting data of others and results from the first-principles simulation based on the local density approximation. The simulated equations of state of MgSiO3 perovskite at higher temperatures and higher pressures also correspond to the other calculations. In addition, the volume compression data of MgSiO3 perovskite is simulated up to 120 GPa at 300, 900, 2000 and 3000 K, respectively.

MOLECULAR THERMODYNAMIC MODEL FOR FLUIDS CONTAINING ASSOCIATED MOLECULES

With the statistical mechanical theory for square-well-chain fluids as the basic princi-ples,a molecular thermodynamic model including expressions for the Helmholtz function and thecompressibility factor for associated square-well-chain fluids has been developed.Employment ofthe shield-sticky concept enables the corporation of association bonds between in the model.Good agreement with computer-simulation results for dimer-linear quadrimer systems is obtained.Satisfactory correlation for vapor pressures and saturated liquid volumes of pure substances con-taining associated molecules(such as carboxylic acids,alcohols,amines,water,etc.)with four mo-lecular parameters indicates the applicability of the model.

Molecular dynamics study of helium bubble pressure in titanium

In this paper, the pressure state of the helium bubble in titanium is simulated by a molecular dynamics （MD） method. First, the possible helium/vacancy ratio is determined according to therelation between the bubble pressure and helium/vacancy ratio; then the dependences of the helium bubble pressure on the bubble radius at different temperatures are studied. It is shown that the product of the bubble pressure and the radius is approximately a constant, a result justifying the pressure-radius relation predicted by thermodynamics-based theory for gas bubble. Furthermore, a state equation of the helium bubble is established based on the MD calculations. Comparison between the results obtained by the state equation and corresponding experimental data shows that the state equation can describe reasonably the state of helium bubble and thus could be used for Monte Carlo simulations of the evolution of helium bubble in metals.

Molecular Dynamics Study of gases H2,D2 and T2

The classical molecular dynamics simulation has been used to study the equation of state of gas H2, D2 and T2. It has also been investigated that the isotope mass affects on the accuracy of equation of state. Our calculated results show that the classical effect is principal and the isotope mass effects on the equation of state are obvious for the much light gases. At the same time, some useful theoretical data of equation of state for these gases have been provided. It is found that the classical simulation is still effective to the quantum gas. However, the quantum mechanics simulation and the improvement of intermolecular interaction potential are necessary if more accurate computational results are expected.

基于VLW状态方程的氢氟爆轰参数理论分析及数值模拟研究

氟气是具有强氧化性的气体,与氢气会发生剧烈的化学反应,为了得到氢氟爆轰的爆轰参数,基于VLW状态方程与爆轰理论对氟气与氢气在标准状况预混条件下爆轰性能进行了理论计算,当氢气与氟气的摩尔比为1∶1时,计算得到爆压为15.31 MPa,爆温为6 479 K。利用materials studio与CP/2K分子动力学模拟计算了氢与氟的反应,对现有氢氟反应机理进行了完善。最后通过FLUENT流体动力学仿真软件对摩尔比为1∶1时的氢氟爆轰进行了仿真计算,因爆轰波在管道内多次叠加,所以计算出超压可达46 MPa,高于理论爆压值的15.31 MPa;管道内计算出的温度可达6 800 K,与理论爆温值相差4.6%,对研究氢氟爆轰条件与能量释放规律具有一定借鉴意义。

Semiempirical equations of state of H_(2)O/CO_(2) binary mixtures in graphite nanoslits

The existence of confining walls limits the prediction accuracy of nanoconfined fluids using macroscopic equations of state(EOSs);moreover,appropriate EOSs for multicomponent mixture fluids in nanoconfined spaces are missing.Here,we derive the EOS of multicomponent mixture fluids confined in nanospaces at high temperatures and pressures,mainly considering the nanoconfinement effect and the competitive adsorption effect between different components.Then,the EOSs are validated through comparison with the molecular dynamics-simulated Pv T data of CO_(2)/H_(2)O mixtures in graphite nanoslits.To consider the above effects,we derive two EOSs via two modeling methods:EOS I is obtained through modification of the actual component occupation volume in the Peng-Robinson equation of state(PR EOS)by fitting the binary component interaction coefficient and the number of adsorbed molecules according to a selectivity coefficient,while EOS II is obtained by considering the decreased pressure of the fluids in PR EOS by adding an attractive term between components and walls.With the simulation results as a benchmark,the two EOSs exhibited good prediction accuracies under low CO_(2) concentrations,and generally,EOS II was more accurate than EOS I.This study fills the gap in the EOSs of nanoconfined mixture fluids,and the obtained equations can help to further describe the thermodynamic properties of confined mixture fluids.

Equations of state for aqueous solutions under mantle conditions

With unique physical and chemical properties, aqueous solutions in the mantle may play important roles for a number of geochemical and geodynamical processes. However, since experimental data available are very limited, people still know little about the aqueous solutions and their interactions with surrounding rocks and melts. From the perspective of thermodynamics, equation of state(EOS) is the key to push forward the modeling of aqueous solutions. Nevertheless, up to now accurate EOSs suitable for the mantle conditions are still in shortage. With discussions over several recognized EOSs, we summarize several ways to enhance the predictability of EOS: utilizing high quality data from molecular simulations, choosing functions with sound physical background, and improving the regression procedures for the empirical parameters. In the meantime, we find that the ion-bearing systems are still the focus of challenges in this area. New developments of experiments and computer simulations effectively deal with these challenges and in-depth understandings of aqueous solutions in the mantle are expected in the near future.

A theoretical form of the Martin-Hou equation of state

A new equation of state is derived from the Barker-Henderson hard-sphere perturbation theory. It has the form similar to the Martin-Hou equation of state. The numerical values of the characteristic constants in the equation can be calculatrd by the method of Martin and Hou. The equation can he used to predictP-V-T properties accurately for fluids when the critical parameters (Tc,Pc, and Vc) and one point on the vapor pressure cure are given. By using the functional relationships between the characteristic constants and the microscopic parameters, the molecular microscopic parameters of the substance can be obtained.

Theory and algorithms for the excited states of large molecules and molecular aggregates

This project aims to attack the frontiers of electronic structure calculations on the excited states of large molecules and molecular aggregates by developing novel theoretical and computational methods. The methodology development is especially based on the time-dependent density functional theory (TDDFT) and valence bond (VB) theory, and is expected to be computationally effective and accurate as well. Research works on the following related subjects will be performed: (1) The analytical energy-derivative approaches for electronically excited state within TDDFT will be developed to reduce bypass the computational costs in the calculation of molecular excited-state properties. (2) The ab initio methods for electronically excited state based on VB theory and hybrid TDDFT-VB method will be developed to overcome the limitations of current TDDFT in simulating photophysics and photochemistry. (3) For larger aggregates, neither ab initio methods nor TDDFT is applicable. We intend to build the effective model Hamiltonian by developing novel theoretical and computational methods to calculate the involved microscopic physical parameters from the first-principles methods. The constructed effective Hamiltonian is then used to describe the excitonic states and excitonic dynamics of the natural or artificial photosynthesized systems, organic or inorganic photovoltaic cell. (4) The condensed phase environment is taken into account by combining the developed theories and algorithms based on TDDFT and VB with the polarizable continuum solvent models (PCM), molecular mechanism (MM), classical electrodynamics (ED) or molecular dynamics (MD) theory. (5) Highly efficient software packages will be designed and developed.

Vapor-liquid equilibria of CH_4, CO_2 and their binary system CH_4 + CO_2: A comparison between the molecular simulation and equation of state

An accurate knowledge about phase behaviors of CH4, CO2 and their binary mixture is crucial in fields of natural gas liquefaction and refrigeration applications. In this work, two all-atom force fields of TraPPE-EH and EMP2 were used for the components CH4 and CO〉 respectively. Then the vapor-liquid equilibria （VLE） of CH4, CO2 and their binary system were calculated via the NVT- and NpT Gibbs Ensemble Monte Carlo Simulations. Meanwhile the traditional method using Equation of State （EoS） to correlate the VLE properties was also investigated. The EoSs considered in this work were three classic cubic RK, SRK, PR and another advanced molecular-based PC-SAFT equations. For pure components, both molecular simulations and the PC-SAFT EoS could obtain satisfactory predictions for all the saturated properties. However, the saturated liquid densities calculated by the cubic EoSs were not so good. It was also observed that the TraPPE-EH force field had a good representation for CH4 molecule, while the EMP2 force field was not enough accurate to represent CO2 molecules. For the mixture CH4 ＋ CO2, SRK and PR showed the best predictions for the saturated pressure-component property, while good results were also obtained via molecular simulations and PC-SAFT EoS. It was suggested that special combining rules or binary interaction parameters were important to obtain enough accurate prediction of the mixed phase behavior. Compared with the cubic EoS, the PC-SAFT and molecular simulation method showed better adaptabilities for both the pure and mixture systems. Besides, the accurate molecular parameters used in the PC-SAFT and molecular simulations could bring about direct and deep understanding about the molecular characteristics.

Theoretical methods for excited state dynamics of molecules and molecular aggregates

This contribution provides a summary of proposed theoretical and computational studies on excited state dynamics in molecular aggregates, as an important part of the National Natural Science Foundation (NNSF) Major Project entitled "Theoretical study of the low-lying electronic excited state for molecular aggregates". This study will focus on developments of novel methods to simulate excited state dynamics of molecular aggregates, with the aim of understanding several important chemical physics processes, and providing a solid foundation for predicting the opto-electronic properties of organic functional materials and devices. The contents of this study include: (1) The quantum chemical methods for electronic excited state and electronic couplings targeted for dynamics in molecular aggregates; (2) Methods to construct effective Hamiltonian models, and to solve their dynamics using system-bath approaches; (3) Non-adiabatic mixed quantum-classic methods targeted for molecular aggregates; (4) Theoretical studies of charge and energy transfer, and related spectroscopic phenomena in molecular aggregates.

First-Principles Study of Orthorhombic Perovskites MgSiO3 up to 120 GPa and Its Geophysical Implications

High-pressure behaviour of orthorhombic MgSiO3 perovskite crystal is simulated by using the density functional theory and plane-wave pseudopotentials approach up to 120 GPa pressure at zero temperature. The lattice constants and mass density of the MgSiO3 crystal as functions of pressure are computed, and the corresponding bulk modulus and bulk velocity are evaluated. Our theoretical results agree well with the high-pressure experimental data. A thermodynamic method is introduced to correct the temperature effect on the O-K first-principles results of bulk wave velocity, bulk modulus and mass density in lower mantle PIT range. Taking into account the temperature corrections, the corrected mass density, bulk modulus and bulk wave velocity of MgSiO3-perovskite are estimated from the first-principles results to be 2%, 4%, and 1% lower than the preliminary reference Earth model （PREM） profile, respectively, supporting the possibility of a pure perovskite lower mantle model.

Interaction potential and thermo-correction to the equation of state for thermally stable Schwarzschild anti-de Sitter black holes

The microscopic structure of black holes remains a challenging subject. In this paper, based on the well-accepted fact that black holes can be mapped to thermodynamic systems, we make a preliminary exploration of the microscopic structure of the thermodynamically stable Schwarzschild anti-de-Sitter(SAdS) black hole. In accordance with the number density and thermodynamic scalar curvature, we give the interaction potential among the molecules of thermodynamically stable SAdS black holes and analyze its effectiveness. Moreover, we derive the thermo-correction to the equation of state for such black holes that arises from interactions among black-hole molecules using virial coefficients.

Equation of State for Shock Compressed Xenon in the Ionization Regime:ab Initio Study

Quantum molecular dynamic (QMD) simulations have been applied to study the thermophysical properties of liquid xenon under dynamic compressions. The equation of state (EOS) obtained from QMD calculations are corrected according to Saha equation, and contributions from atomic ionization, which are of predominance in determining the EOS at high temperature and pressure, are considered. For the pressures below 160 GPa, the necessity in accounting for the atomic ionization has been demonstrated by the Hugoniot curve, which shows excellent agreement with previous experimental measurements, and three levels of ionization have been proved to be sufficient at this stage.

温稠密氦的状态方程、热输运性质和辐射不透明度的理论研究

温稠密状态下氦元素广泛存在于核聚变内爆过程和宇宙星体中,其热力学性质和辐射输运参数等物质特性在聚变实验设计和星体结构演化研究中起着至关重要的作用.本文采用充分考虑温稠密物质中电子离子碰撞物理效应的量子郎之万分子动力学方法,通过模拟宽广温度密度区域氦离子和电子的响应特性,构建了温度在10-60 kK,密度为1-24 g/cm^(3)范围内温稠密氦的状态方程数据库和电子热导率数据库,并计算了该温度密度下温稠密氦的辐射不透明度.本文的计算结果可以为聚变物理研究和很多基本天体物理问题建模提供必要的输入参数.

温稠密氢氘混合物热力学、光学和电子输运特性理论研究

温稠密状态下的氘氚混合物是惯性约束聚变必然要经历的中间物质状态.其在温稠密状态下的热力学特性和输运参数在聚变实验设计研究中起着重要作用.但由于氚具有放射性,实验中可以用氢氘混合物来模拟替代研究氘氚混合物.本文采用了密度泛函理论分子动力学模拟和基于自洽流体变分理论的化学模型,构建了氢氘混合物在0~200 GPa和300~10000 K宽范围状态方程和声速数据库.此外,结合Kubo–Greenwood公式计算得到了氢氘混合物电子电导率以及光学反射率和折射率,建立起混合物绝缘–金属化相变与实验上易探测的光学物理量反射率和折射率之间的联系,为从实验角度探索氢及其同位素的金属化提供了理论指导.