(742c) Prediction of the Crystal Packing of Triazine-Triamine-Trioxide (MTO) and Triazine-Trinitro-Trioxide (MTO3N) Reaching High Densities

Authors: 
Naserifar, S., California Institute of Technology
Goddard III, W. A., California Institute of Technology
Zybin, S., California Institute of Technology

Prediction of the Crystal Packing of Triazine-Triamine-Trioxide (MTO) and Triazine-Trinitro-Trioxide (MTO3N), Reaching High Densities

The design of safe and efficient high-energy materials is of vital importance, due to its widespread civil applications for the use in propellants for carriers, or satellite launch rockets and satellite propulsions system. There is much current interest in being able to predict the performance of new high-energy molecules that have not been synthesized previously. Computational methodologies have an edge over synthetic work for high-energy materials, because it is less expensive financially to investigate the properties of promising molecules, it is less time consuming and, of course, much safer. However, until recently the properties of molecular solids was a challenge to periodic and non-periodic ab-initio calculations. Non-periodic calculations that are able to predict correctly the non-covalent interaction are too expensive computationally, and are constrained to some tens of atoms. The most popular ab initio method, due to its less expensive requirements, is DFT (with scaling of N3 or N4, with N being the number of orbitals), but this does not capture the dispersive forces correctly. Many research groups have added empirical parameters to the various DFT functionals to handle the inaccuracy of DFT methods for capturing the non-covalent forces correctly, but keeping the scalability. We have developed a method that combines non-periodic and periodic high-level QM calculations in order to predict the most stable phase of Triazine-triamine-trioxide (MTO) and Triazine-trinitro-trioxide (MTO3N) which are predicted to have high energy densities. We have explored several different space groups for each of them using Monte Carlo annealing techniques followed by geometry and cell optimization, and selected the best candidates with an initial force field ranking. The best structures were analyzed with the state of the art periodic DFT functional; PBE-D2 that predicts non-covalent interactions near chemical accuracy. The most stable isomers have the highest densities ever reported for an energetic material with densities of 2.0 to 2.1 g/cm3. Thus, a method has been developed that can be expanded to predict the stability and packing of other molecular solids where the non-covalent interactions are crucial.

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