Parameterization And Crystal Structure Prediction Of The Polarizable Force Field Of RDX And CL-20 Energetic Molecules

Energetic materials are widely used in military and civil fields due to their highenergy properties.The physical and chemical properties of energetic materials,such as sensitivity,detonation velocity,detonation pressure and energy,are not only related to the chemical composition and geometry structure of energetic molecules,but also related to the crystal stacking structures of molecules.In most cases,high-energy molecules have high sensitivity and poor safety.Although it is difficult to reduce the safety of molecules from the molecular level,if the molecular packing is effectively controlled,the safety of materials possibly can be significantly improved.Therefore,it is of great significance to predict the possible crystal structure of energetic molecules before synthesizing high-risk energetic molecules.The challenges of molecular crystal structure prediction mainly lie in two aspects:1)global search of cell structure in high-dimensional freedom.The high-dimensional degrees of freedom,including six unit-cell parameters(a,b,c,?,?,?),three molecular orientational angles(?,?,?),230 space symmetries and degrees of freedom of molecular conformation,is almost impossible to scan all freedoms,so it is usually necessary to develop some efficient algorithms such as lowering dimension scaning,effective MonteCarlo samplings and genetic algorithm to achieve the global optimization.2)Rapid and accurately ranking the possible unit cells.The calculation methods of crystal structure energy include quantum chemistry(QM)method and molecular force field(MM)method.Although QM can provide an accurate method to calculate the energy of crystal,its cost is too high,which limits its large-scale application in the prediction of molecular crystals.MM is a low-cost method and can be used in the prediction of crystal structure.However,the accuracy of MM often strongly depends on the force field parameters.It is very important to develop high-precision molecular force field for crystal structure ranking and evaluation.In this theisis,QM calculations are used to obtain the non-empirical polarizable force field for energetic molecules,which is called as EMS-PFF.Combined with genetic evolution USPEX program,EMS-PFF is used to predict the crystal structure of energetic materials.The thesis mainly includes the following four charpters:1.The procedure of parametrization of the energetic molecule specific polarizable force field(EMS-PFF).Based on QM calculations,the electrostatic and polarization parameters in EMS-PFF are obtained,including atomic polarizability by finite field method,anisotropic atomic multipole moment electrostatic parameters by GDMA and intramolecular force field parameters by paratools.The results show that the anisotropic atomic multipole moments can better repudcues QM's electrostatic potential on the molecular vdW surface,the atomic polarizability obtained by finite field can well repeat the anisotropic molecular polarizability,and the molecular internal force field parameters calculated by QM can well maintain the molecular rigid geometry.2.Taking RDX as an example,the single crystal structure of ?-RDX molecule is predicted by USPEX.Under room temperature,RDX molecule just exists as ?-RDX crystal morphology.In this chapter,on the basis of the conformation of ?-RDX in experimental crystal,EMS-PFF are obtained from QM calculations mentioned before.And the crystal structure of ?-RDX is successfully predicted by EMS-PFF combined with USPEX program.The predicted structure with the lowest energy is consistent with the experimental crystal structure.It shows that the non-empirical EMS-PFF do well describe the intermolecular interactions in the RDX molecular system.In addition,the general standard force field(Dreiding,Compass)based on isotropic point charge is applied to predict the crystal structure of ?-RDX.The results show that the charge-based general force field combined with rigid molecular optimization always find some plausible structures with lower energy than the experimental crystal,and the anisotropic electrostatic potential is the very important factor in predicting the crystal structure of rigid molecules.3.In practice,energetic molecules may co-exist in different molecular conformations,which will lead to the different polymorphies strucures.In this chapter,EMS-PFF of CL20 with different polymorphies(?-,?-and ?-)was obtained by QM method,and the crystal structures of three crystals were predicted by USPEX.The results show that the anisotropic electrostatic parameters are the key factor to predict the crystal structure successfully,while the atomic polarizability only slightly affects the crystal structure.The electrostatic potential of the atomic multipole moment without the electronic polarization possibly is a better way to accelerate the crystal structure prediction.4.TNT as an example,the predication of crystal structure with motif greater than 1(z'>1)was explored.At present,most of the crystal structure prediction is performed under rigid molecule approximation,which require the motif of crystal less than 1.It is also important to predict the crystal structure whose z'>1 for a wider range of molecular crystals or co-crystal structures.In this chapter,we use EMS-PFF to tentatively predict the crystal structure of TNT.The results show that the EMS-PFF are failour to predict the crystal structure of TNT.The reason may be due to the fact that present EMS-PFF can not correctly describe the intramolecular conformational energy when the molecule deviates from the balanced geometry.In addition,the effect of molecular conformation on atomic multipole moments may be very significant.