The N 4 (Td) Intermolecular Force Field Parameters And Their Density Prediction Are Optimized Based On QM Non-bonding Binding Energy

Searching and screening the new type of green high-energy-density materials （HEDMs） have been a challenging human goal for military/civilization purpose. Pure polynitrogen compounds have been placed on a key candidate of HEDMs due to their environmentally friendly decomposing products N₂, potential high laoding density and high positive heat of formation. In this paper, we took the N₄（Td） as the object to develop it intermolecular interaction potential parameters based on the quantum mechanics calculations. And we use the reparameterized atom-atom potential functional to evaluate the loading density. In the process of force field parameterization, Monte Carlo coupled with rotational isomeric state method （MC/RIS） is used to construct molecular amorphous structures to obtain the large amount of possible full configurational dimers. Then, the intermolecular interaction energy of these dimers are calculated using quantum mechanics method as accurateas possible to fit the vdW force field parameter of N₄（Td） system. At last, the reparamerterized 12-6 LJ potential is used to predict the density at OK and at the room temperature. Details as follows:In chapter one, I briefly introduced the current research situations of energetic materials, the evaluation criterions of its performance and the challenges for theoretical design HEDMs. As a key estimated index for HEDMs, we revisit the most of method to predict the loading density and discuss these method respective advantages and disadvantages. Finally we determine that the atom-atom potential method is suitable method to predict the total novel compound N₄.In chapter two, I introduce some theoretical methods related to predication of loading density, including quantum mechanics（QM）, molecular mechanics（MM） and molecular dynamics（MD）. In MM part, we mainly discuss the force field potential functional in particular, intermolecular potential. In MD processes, the ensemble concept are introduced and the control method of temperature and pressure also briefly explained.In chapter three, we mainly introduced some key challenges about parameterization the atom-atom potential based on quantum mechanics such as how to get accurate intermolecular interaction with suitable cost, dimer configuration sampling on full potential surface, the choose of intermolecular interaction functions, three-body/many body effect on potential parameters. For nonbonding interactions QM calculations, we firstly chose A24 database as object, try different cheaper methods to calculate the 24 small molcules interactions and compared them with golden standard CCSD（T）/CBS to screen more cheaper method to evaluate the nonbonding interactions. Next we choose some face-to-face dimer configurations as testing set to rationalize MP2 with small basis set cc-pvdz owning a good performance to nonbonding interaction and with small computer costs.In chapter four, the atom-atom force field for N₄（Td） including intermolecular and intramolecular potential are parameterized and it is applied to predict the loading density. As for force field parameterization, we firstly based on a MC/RIS dimer sampling to testify different LJ potentials. According to the Root Mean Squared Error （RMSE） for different LJ potential contour plots on vdW depth and length, I find that 12-6 LJ functional can well describethe intermolecular interactions. Then the different MC/RIS samplings due to variable sampling initial density, initial temperature, and center-to-center distance are checked to effect on stability 12-6 LJ parameters. In addition, the three body effect also is discussed in this thesis. The results suggest that MC/RIS method can readily obtain a stable fitted parameters for LJ potential with weak dependence on these factors. All the possible lowest binding energy crystal structures are obtained by using the Polymorph module in Materials Studio on the basis of reparameterized Dreiding force field, the crystal density at OK is around 1.70g·cm-3 Moreover the loading density of room temperature is predicted by a MD simulation at 298K and 1 atm. using our reparameterized 12-6 LJ potential. The simulation results show that N₄Td) exists in the form of gas phase at room temperature. But it can become condensed phase when the pressure is increased to 55 atm. at 298K and when the temperature is reduced to 255K under latm.