  ReaxFF provides a method to model reactive chemical systems in large-scale
molecular dynamics simulations. Here, we developed ReaxFF parameters for
phosphorus and hydrogen to give a good description of the chemical and
mechanical properties of pristine and defected black phosphorene. ReaxFF for
P/H is transferable to a wide range of phosphorus and hydrogen containing
systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated
phosphorene, phosphorus clusters and phosphorus hydride molecules. The
potential parameters were obtained by conducting unbiased global optimization
with respect to a set of reference data generated by extensive ab initio
calculations. We extend ReaxFF by adding a 60{\deg} correction term which
significantly improves the description of phosphorus clusters. Emphasis has
been put on obtaining a good description of mechanical response of black
phosphorene with different types of defects. Compared to nonreactive SW
potential [1], ReaxFF for P/H systems provides a huge improvement in describing
the mechanical properties the pristine and defected black phosphorene and the
thermal stability of phosphorene nanotubes. A counterintuitive phenomenon is
observed that single vacancies weaken the black phosphorene more than double
vacancies with higher formation energy. Our results also show that mechanical
response of black phosphorene is more sensitive to defects for the zigzag
direction than for the armchair direction. Since ReaxFF allows straightforward
extensions to the heterogeneous systems, such as oxides, nitrides, ReaxFF
parameters for P/H systems build a solid foundation for the reactive force
field description of heterogeneous P systems, including P-containing 2D van der
Waals heterostructures, oxides, etc.
