  In recent years, certain molecular crystals have been reported to possess
surprising flexibility by undergoing significant elastic or plastic deformation
in response to mechanical loads. However, despite this experimental evidence,
there currently exists no atomistic mechanism to explain the physical origin of
this phenomenon from numerical simulations. In this study, we investigate the
mechanical behavior of three naphthalene diimide derivatives, which serve as
representative examples, using direct molecular dynamics simulations. Our
simulation trajectory analysis suggests that molecular rotational freedom is
the key factor in determining a crystal's mechanical response, ranging from
brittle fracture to elastic or plastic deformation under mechanical bending.
Additionally, we propose a rotation-dependent potential energy surface as a
means to classify organic materials' mechanical responses and identify new
candidates for future investigation.
