  The single-layer black phosphorus is characteristic for its puckered
configuration that possesses the hinge-like mechanism, which leads to the
highly anisotropic in-plane Poisson's ratios and the negative out-of-plane
Poisson's ratio. We reveal that the hinge-like mechanism can be attributed to
the angle-angle cross interaction, which, combined with the bond stretching and
angle bending interactions, is able to provide a good description of the
mechanical properties in the single-layer black phosphorus. We also propose a
nonlinear angle-angle cross interaction, which follows the form of the
Stillinger-Weber potential and is advantageous for molecular dynamics
simulations of single-layer black phosphorous under large deformations.
