All-atom, non-empirical, and tailor-made force field for α-RDX from first principles

A general strategy is presented to develop an all-atom, non-empirical, and tailor-made force field (NETMFF) for high explosives (HEs). The central part of the strategy is a self-consistent force field (SCFF) optimization technique. The consistence of the force field is ensured by iterating the parameterization procedure. The generation of reliable ab initio reference data for optimizing the NETMFF parameters and the SCFF technique are discussed in detail. Starting with the crystal structure obtained from either experiment or crystal structure prediction (CSP), NETMFF can be developed only by first principles, including conventional DFT, the periodic DFT-D model, and the SAPT(DFT) plus Williams-Stone-Misquitta (WSM) methods. The development strategy of NEMTFF has been applied to α phase hexahydro-1,3,5-trinitro-1,3,5-triazine (α-RDX), an important high explosive. The force field obtained for α-RDX can yield the correct geometries for all RDX conformers found by the DFT calculations in a good way, and also can identify the transition states obtained from the DFT calculations by force field-based vibration analysis. More importantly, it can accurately predict densities of high explosives under the environmental conditions to which they are often subjected, a long-standing issue in the field of energetic materials. The parameterization strategy described in this paper can be easily generalized toward other known HEs or the new HEs whose crystal structures can be obtained by CSP.