  Amorphous silica ($a-SiO_2$) is a widely used inorganic material.
Interestingly, the relationship between the local atomic structures of
$a-SiO_2$ and their effects on ductility and fracture is seldom explored. Here,
we combine large-scale molecular dynamics simulations and machine learning
methods to examine the molecular deformations and fracture mechanisms of
$a-SiO_2$. By quenching at high pressures, we demonstrate that densifying
$a-SiO_2$ increases the ductility and toughness. Through theoretical analysis
and simulation results, we find that changes in local bonding topologies
greatly facilitate energy dissipation during plastic deformation, particularly
if the coordination numbers decrease. The appearance of fracture can then be
accurately located based on the spatial distribution of the atoms. We further
observe that the static unstrained structure encodes the propensity for local
atomic coordination to change during applied strain, hence a distinct
connection can be made between the initial atomic configurations before loading
and the final far-from-equilibrium atomic configurations upon fracture. These
results are essential for understanding how atomic arrangements strongly
influence the mechanical properties and structural features in amorphous solids
and will be useful in atomistic design of functional materials.

Mesoscale and Nanoscale Physics;Materials Science