Research And Application Of Multi-scale Coarse Grain Algorithm Based On Intermolecular Interaction

With the development of computer technology,computer simulation has become an important technique to study the structure and performance of materials and biomolecules.Owing to limited computing capacity,molecular dynamics based on all-atom has some limitations on enlarging the simulation system and extending the time scale.In terms of ensuring both computational efficiency and simulation accuracy,the multi-scale simulation method has significant advantages.The connection of different scales is a key point to the multi-scale method.Therefore,a coarse-grained simulation method spanning multi-scale has obviously advantages to study the relationship between the structure and performance of substances at different scales.For the mesoscopic scale hybrid system,because of the boundary effect,its simulation is more complicated than the single component system,which is one of the hotspots of current research.In this thesis,a "quasi-bond" based coarse-grained method(Quasi-Bond Coarse-Grained,QBCG)is innovatively proposed to establish a coarse-grained model of molecular crystal materials.First,the crystal molecules are coarse-grained into several "beads",each bead is connected by quasi-bonds to form a crystal structure.Based on the coarse-grained model,the proper structure of crystals is maintained under extremely high-pressure conditions.Then the hierarchical multi-scale method is used to obtain the coarse-grained force field corresponding to the crystalline material.Furthermore,a concurrent multi-scale method is used to obtain a mixed coarse-grained force field for composites with different coarse-grained resolutions.Using the above-mentioned multi-scale coarse-grained methods,multi-scale simulations on TATB crystals and polymer-bonded explosive composites based on TATB are performed,and conclusions are as follows:1.In the coarse-grained simulation of TATB,calculation results of thermal expansion coefficient show that the expansion of TATB crystals is anisotropic,which the expansion rates of a and b axis are similar and the c axis expansion rate is the largest;Using the three equation of state(EOS)of Brich-Murnaghan,Murnaghan and Vient to fit the pressure-volume(P-V)curve of TATB,it is found that the Birch-Murnaghan EOS fits best with experimental results.Therefore,Birch-Murnaghan EOS can be used to describe the isothermal compression behavior of TATB.Also,the elastic modulus of TATB calculated by the isothermal compression method and the Rankine-Hungoniot equation fitted by the P-V curve are both consistent with the results in previous references.Results show that the coarse-grained method we established is feasible to study molecular crystals.2.After adding polymer F2314 to TATB,the thermal expansion coefficient of the composite is significantly reduced and the sensitivity to the external environment is also lower at high temperatures;The obvious decreasing of the elastic modulus indicates that the stiffness of the composite is reduced,while the flexibility is increased.Thus,the composite becomes safer during the vibration and impact loading.3.Finally,using the QBCG method,we calculate the thermal expansion coefficient,P-V curve and EOS of RDX,an energetic material.Simulation results are consistent with previous experimental and simulation results.it is proved that the multi-scale coarse-grained method proposed in this thesis is portable;Comparing with the all-atom molecular dynamics method,the QBCG method has a computational efficiency of more than 2 times magnitude.