Search for the Decomposition Process of 2,4,6-Trinitrotoluene by an Evolutionary Algorithm

In this paper, we explored stable states in the system of 2,4,6-trinitrotoluene (TNT) crystal with a few additional hydrogen radicals (H_add's) using a structure-search scheme based on first-principles calculations and an evolutionary algorithm (EA) to get insights into the decomposition process of TNT. We introduced three evolutionary operators acting on H_add's and transforming only local structures of TNT molecules: "displacement", "permutation", and "mating". We searched for stable structures by increasing the number of H_add's (n) from 1 to 2, 3, 4, 6, and 8 and constructed a convex-hull diagram for the formation energy from solid TNT and solid hydrogen. We showed that the system of n = 6 had the largest energy reduction, in which five of the eight TNT molecules in the calculation cell were transformed into NO, H₂O, C₂H₃N, C₂NO₃H₃, C₈N₂O₄H₇, C₉N₂O₈H₅, and C₁₄N₇O₁₂H₁₁. Analysis of the structural transformations observed during the EA search indicates that (1) the H_add's approaching the TNT molecules react with C, forming a six-membered ring, and with N and O in nitro groups, leaving the TNT molecules as NO, H₂O, C₂H₃N, and C₂NO₃H₃, and (2) the partially decomposed TNT molecules are bonded to one another via C, N, and O.