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RDX是环三甲撑三硝胺的简称,又名黑索金,是军民领域常用的高能炸药。生成焓是炸药的基本热力学性质,是其含能高低的标志,从理论上计算分子结构并预测其生成焓对含能材料的分子设计和品优炸药的筛选具有重要意义。在众多计算方法中,DFT采用大型基组计算目标分子的全电子,结果精确,可信度高,缺点是需时长,耗费计算资源;基于半经验法的从头算法是对Hartree-Fock方程的近似求解,耗费机时少,但结果可信度低;分子力场(OFF)基于牛顿经典力学,以原子间的相互作用势函数(或称力场)来描述分子内和分子间的作用力,计算速度非常快,可计算大分子或高分子材料的物理性质,但在计算分子结构方面效果不太理想。本文用密度泛函理论(DFT)与开放力场(OFF)相结合的方法,计算高能炸药——黑索金(RDX)的分子结构和气相生成焓,在保证计算精度的同时节约了时间。在构象搜索阶段,分别采用分子力学(MM)和分子动力学(MD)方法,实现对目标分子的结构驰豫和模拟退火,在分子的几何结构优化阶段,则采用DFT GGA-BLYP法。分析优化RDX分子的键长与键角表明,RDX分子结构的计算结果与实验测定基本吻合。设计反应路径,并利用DFT GGA-BLYP计算出RDX的气相生焓为188.604 kJ/mol,与实验测定值191.444 kJ/mol接近。
RDX is the abbreviation of cyclotrimethylene trinitramine, also known as black gold, is commonly used in military and civilian areas of high explosives. The enthalpy of formation is the basic thermodynamic property of explosives and is a sign of its energy content. It is of great significance to calculate the molecular structure theoretically and predict its enthalpy of formation for the molecular design of energetic materials and screening of high explosive. Among many calculation methods, DFT uses large basis sets to calculate the total electron of the target molecule, the result is accurate and the credibility is high. The disadvantage is that it takes time and consumes computing resources. The ab initio algorithm based on the semi-empirical method is approximate to the Hartree-Fock equation But the result is low in credibility. The molecular force field (OFF) is based on Newtonian classical mechanics and describes the intramolecular and intermolecular forces based on the potential function of atoms (or force field) Computational speed is very fast, can calculate the physical properties of macromolecules or polymer materials, but the effect is not ideal in the calculation of molecular structure. In this paper, the molecular structure and enthalpy of vapor phase formation of high explosive RDX were calculated by the combination of density functional theory (DFT) and open force field (OFF), which can save the time while ensuring the accuracy of calculation. In the conformational search phase, molecular dynamics (MD) and molecular dynamics (MD) methods were used respectively to achieve the structural relaxation and simulated annealing of target molecules. DFT GGA-BLYP method was used in the optimization of molecular geometry. Analysis and optimization of the RDX molecule bond length and bond angle show that the RDX molecular structure of the calculated results and experimental determination of the basic agreement. The reaction path was designed and the enthalpy enthalpy of RDX calculated by DFT GGA-BLYP was 188.604 kJ / mol, which was close to the experimental value of 191.444 kJ / mol.