Using Molecular Mechanics Force Field Method To Predict Thermodynamic Properties And State Function Under Extreme Condition

In this paper, molecular mechanics force field (hereinafter referred to as force field) method and the simulation method in the calculation of thermodynamic properties of fluids in industrial applications are involved. Our goal is mainly focus on the development of classical molecular force field and to use classical molecular simulation methods to solve practical problems: the development of accurate molecular mechanics force field under the philosophy of TEAM force field; Secondly, we combined a variety of simulation methods to predict diversity of thermodynamic properties at the same time under the premise of a unified field; Third, to use molecular dynamics method to predict the state function of the system which is under extreme conditions, such as shock wave is applied; Fourth, to study the influence of the force field parameter on the complex fluid system.The research ideas are as following: force field development is the foundation of this work. Accurate force field parameters are the basic for accurately simulating systems. The force field development is under the philosophy of TEAM force field. The clear physical background of Fitting process guarantees the reasonability and the reliability of the force field parameters. Then we focus on the practical application of classical simulation. To predict industrial fluid properties is a promising application field where molecular simulation method would play increasingly import role. Under the premise of a unified force field, we can forecast a wide range of physical and chemical properties at the same time. At the same time, an accurate prediction of thermodynamic properties of a variety is also the reflection of the reliability of TEAM force field parameters. Research on energetic material TATB demonstrated one of the superiority of molecular simulation, that is, in extreme conditions, the experimental means to be very difficult to achieve if not impossible to implement, and the molecular simulation method is a suitable alternative. Finally, the research on surfactant system reveals that, on the one hand, molecular simulation methods was adopted to solve the key issues of the popular area , the surface-active agent, on the other hand, investigation on this complex systems would be helpful to the development mixture field.The conclusion can be summarized as follows:1. Our first target is to develop reliable and accurate force field, which can precisely describe the chemical and physical state of the research system, under the TEAM philosophy. The guiding ideology of the TEAM force field development process is transferability, extensibility, accuracy, and modularized. In the TEAM force field method, the definition of the atom type is basing on the structure of the atom, which can guarantee the accuracy and completeness of both molecular dynamics simulation and the force field parameter transfer. A systematic fitting progress can ensure that the obtained molecular field can reproduce the molecular structure, vibration frequencies, dihedral angle rotation barrier and other gas properties. To capture the nature of van der Waals parameters, we adopted the test particle method to derive the initial van der Waals parameters. This force field fitting method should be very helpful to develop accurate force field parameters in the case of the force field deficiency.2. We used one molecular mechanics force field to predict a wide range of thermodynamic, kinetic properties. The coupling between the accuracy of molecular field and confidence level of simulation method is one of the most serious obstacles in investigating the ability of classical simulation in area of predicting the thermal and dynamic properties. In this dissertation, we systemically studied the advantages and limitations of a wide variety of computing methods. Basing on this, we have adopted one unified molecular mechanic force field to forecast critical nature of vapor-liquid equilibrium, and the properties derived from fluctuation such as the constant pressure heat capacity, coefficient of thermal Expansion, compression factor, the dynamic properties, such as shear viscosity, diffusion coefficient, and thermal conductivity and so on. This provides a worthy example concerning on using classical force field to calculate industrial fluid parameters.3. The state parameter under extreme conditions is difficult to obtain by experimental method. Using molecular simulation methods is one of the ways to solve this problem. In this dissertation, we took triaminotrinitrobenzene (TATB) as an example to study the state function of the system under shock wave. In this work, we adopted TEAM force field methods to develop reasonable TATB classical molecular mechanics force field parameters. On that basis, we implemented the AE-EOS (Adaptative Erpenbeck equation of state) reported in the literature to simulate TATB Hugoniot curve before the chemical reaction. The predicted state function of TATB system under shock wave by classical molecular dynamics simulations have compares well with the experimental data.4. We calculated the surface tension of the air/surfactant/water system and oil/surfactant/water system. The study found that for pure state system, as long as the force field is suitable classical molecular dynamics simulation can accurate predict the thermal and dynamic properties of the system. However, due to the complexity of the mixed fluid system and the interface system, in the research area of classical molecular dynamics there are many unresolved issues. Among these, force field should be the most serious one. In this paper, we took the air/surfactant/water system and oil/surfactant/water system as an example to calculate surface tension, mainly concerning on the influence of force field parameters on the surface tension calculation. From the simulation results, we found that the surface tension of the mixture system showed high sensitivity to the force field parameter.