New Nonreactive Force Field for Accurate Molecular Dynamics Simulations of TATB at Extreme Conditions

Insensitive
high explosives based on TATB (1,3,5-triamino-2,4,6-trinitrobenzene)
are needed in applications when safety is of paramount importance,
but the basic material properties that give rise to its insensitivity
are not fully understood. Molecular dynamics modeling using empirical
force fields (FFs) has been the main route to characterize many complicated
dynamical properties of TATB single crystal, but these FFs have not
been comprehensively tested at extreme conditions typical of detonation.
We collect a benchmark data set of (quasi)static TATB physical properties
as determined by experiments and electronic structure calculations
and apply this data set to validate four existing TATB FFs along with
a new TATB FF that we develop here and denote as the CEA-LLNL-Missouri
(CLM) FF. Benchmark data include vibrational spectra, the TATB crystal
temperature–pressure–volume equation of state and lattice
parameters, properties of TATB crystal polymorphs and transitions
to the gaseous and liquid states, dimer energy landscapes, the pressure-dependent
elastic tensor, and the energy landscape for inelastic deformation
via sliding of TATB crystal layers. As a general assessment, we find
that the two existing nonreactive FFs are more accurate in describing
TATB’s physical properties compared to the two variants of
the ReaxFF reactive FF considered. The new CLM FF is found to consistently
yield similar or better agreement with experiments and electronic
structure theory than any of the existing FF models, and it presents
a distinct improvement in accurately modeling TATB elasticity and
equation of state. This work is expected to help improve the accuracy
of FF-based modeling of complicated dynamic responses that ultimately
govern the safety and performance characteristics of this material.