Polymorphism
is universal in energetic crystals and brings much complexity in revealing
the underlying mechanism for energetic materials against external
stimuli. This work comprehensively studies molecular conformers (MCs),
molecular stacking, and related MC energy (MCE) and lattice energy
(LE) of polymorphs of six common energetic materials (EMs), including
2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20),
1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), 1,3,5-trinitro-1,3,5-triazinane
(RDX), 2,2-dinitroethylene-1,1-diamine (FOX-7), 2,4,6-trinitrotoluene
(TNT), and pentaerythritol tetranitrate (PETN). We find the dominance
of MCE or LE in determining the most stable polymorph at common conditions
is variable; i.e., LE dominates CL-20, RDX, PETN, and FOX-7, while
MCE governs HMX, and for the two polymorphs of TNT at below −150
°C, they have almost the same stability without a difference
in either MCE or LE. The variability of the dominance is responsible
for the current difficulty in crystal structure predictions. Moreover,
the temperature elevation reduces the molecular volume (V
m) of β-FOX and PETN-II by weakening intermolecular
interactions or crystal field effects to relax molecules to be smaller
ones. Besides, the high pressure of several GPa mainly shortens the
intermolecular distances, rather than compresses V
m. Finally, the heat-induced polymorphic transformation
of FOX-7 from α- to β- and γ-forms allows it to
be more and more closely face-to-face π–π stacked
and serves as a reason for its low impact sensitivity. All these findings
are expected to deepen insight into the response mechanism of EMs
against external stimuli.