Phase Transition in Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under Static Compression: An Application of the First-Principles Method Specialized for CHNO Solid Explosives

The first-principles
method is challenged by accurate prediction
of van der Waals interactions, which are ubiquitous in nature and
crucial for determining the structure of molecules and condensed matter.
We have contributed to this by constructing a set of pseudopotentials
and pseudoatomic orbital basis specialized for molecular systems consisting
of C/H/N/O elements. The reliability of the present method is verified
from the interaction energies of 45 kinds of complexes (comparing
with CCSD­(T)) and the crystalline structures of 23 kinds of typical
explosive solids (comparing with experiments). Using this method,
we have studied the phase transition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
(HMX) under static compression up to 50 GPa. Kinetically, intramolecular
deformation has priority in the competition with intermolecular packing
deformation by ∼87%. A possible γ → β phase
transition is found at around 2.10 GPa, and the migration of H2O has an effect of kinetically pushing this process. We make
it clear that no β → δ/ε → δ
phase transition occurs at 27 GPa, which has long been a hot debate
in experiments. In addition, the P–V relation, bulk modulus, and acoustic velocity are also
predicted for α-, δ-, and γ-HMX, which are experimentally
unavailable.