| RDX, an important explosive with high characters and properties, is an energetic ingredient that is used in various solid propellants and explosives. Understanding its structure and the complex physicochemical process that underlie the combustion of this material can lead to method for modifying the propellant and explosive formulations in order to obtain better ignition, combustion, or sensitivity properties.On the other hand, it is well-known that the nonbonding interaction potentials, mainly resulting from van der Waals (vdW) interactions, play a crucial role to understand the structures and properties of gases, liquids, solids, as well as any other materials in condensed phase. It is also an important component of empirical potentials, which are often used in molecular dynamic simulations.In the work, we first calculate the electronic structures and properties of RDX molecule and its intermediate products formed during its decomposition process at B3LYP level using 6-31G(d, p) basis set. We present the geometrical parameters, energies, bond orders, thermochemestries and so on. Using those results, we also calculate the bond dissociation energies and rate constant of reaction for RDX and then study the decomposition mechanism of RDX from bond orders and bond dissociation energies two perspectives, respectively. Results show that the dominant initial reaction channel is the N-NO2 bond rupture, which has the weakest Mulliken bond order and bond dissociation energy for 0.1623 and 157.2718KJ/mol respectively. These are consistent with the experimental results.Secondly, the intermolecular interaction potentials of van der Waals Ar-N2 complex have been studied by ab initio calculations using the single and double excitation coupled cluster [CCSD(T)] theory with perturbative triples correction. The full counterpoise method is applied to correct the basis set superposition error (BSSE). It is found that the T-shaped structure is the most stable conformation with the well depth De of 12.40meV at the minimum distance Rm of 3.70A. The calculated anisotropic values for △Rm, △R0 and △De are 0.56A, 0.54A and 2.68meV, respectively. Compared with those obtained by others, our calculated PES seems to be in better agreement with experiments.Thirdly, the equilibrium structure and intermolecular interaction potentials of water dimer have been calculated using supermolecular many-body perturbation theory at the second-order to fourth-order level. The augmented correlation-consistent basis set aug-cc-pVTZ and the effective midbond functions {3s3p2dlflg} are employed. Basis superposition error is corrected by applying the counterpoise procedure. At the MP2 level with aug-cc-pVTZ basis set, the values of Ro-o and a slightly increase 0.02A and 0.19, respectively, while the magnitude of θ drops 0.013 on the CP-optimization geometry compared with the normal optimization geometry. The MP2 calculations with augmented correlation-consistent basis set combining with effective midbond functions predict the values of Ro-o, and △Ecp, for 2.93A and -4.86kcal/mol respectively, which are in better agreement with experiment. The discrete grid of calculated intermolecular interaction energies is fitted to exp-4.2 potential function. The agreement between the fitting and the ab initio results is excellent.
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