  The distinguishing structural feature of single-layer black phosphorus is its
puckered structure, which leads to many novel physical properties. In this
work, we first present a new parameterization of the Stillinger-Weber potential
for single-layer black phosphorus. In doing so, we reveal the importance of a
cross-pucker interaction term in capturing its unique mechanical properties,
such as a negative Poisson's ratio. In particular, we show that the
cross-pucker interaction enables the pucker to act as a re-entrant hinge, which
expands in the lateral direction when it is stretched in the longitudinal
direction. As a consequence, single-layer black phosphorus has a negative
Poisson's ratio in the direction perpendicular to the atomic plane. As an
additional demonstration of the impact of the cross-pucker interaction, we show
that it is also the key factor that enables capturing of the edge
stress-induced bending of single-layer black phosphorus that has been reported
in ab initio calculations.
