The electron occupations are determined with the Mullikan Population Analysis (MPA) scheme . There are several charge partitioning schemes available, such as the Bader Population Analysis (BPA) , the Hirshfeld Population Analysis (HPA) , and the Natural Population Analysis (NPA) . However, it is only the MPA scheme that provides an orbital-dependent decomposition scheme. In spite of the discrepancies among the schemes (in terms of how the total charge is partitioned among different atoms ), the 'change in electron population per atom' due to applied deformation can be deemed method-independent. Also, the 'change in electron population per orbital' is a reliable estimator of the deformation-dependent electron population. It is widely used to analyze the local atomic properties for a number of bulk, molecular, and defective systems . In addition to describing chemical interactions of molecular systems, the MPA scheme is known to provide reliable mechanical properties (such as hardness in solids with zinc blende structure ). We therefore use the MPA scheme to examine the change in electron occupation in the individual electronic orbitals. Electron density maps to observe electron redistribution are generated using the XCrysden and VESTA software packages, and the atomic configurations showing the bond length variations in the lattice are generated using OVITO . The unit cell of the phosphorene layer is first created with four atoms in a basic simulation cell. The atomic coordinates of the four atoms in the irreducible unit cell are: (1.671, 0.396, 5.243), (0.000, 1.913, 5.244), (0.000, 2.7057, 7.396), and (1.671, 4.222, 7.396). Referring to the upper panel of Fig. , the length of the bonds OA and OB, both of which are lying on the same xyplane, is r 1 = (x 2 -x 1 ) 2 + (y 2 -y 1 ) 2 = 2.25 Å, and the length of the out-of-plane bond, which lies an a plane oriented at an angle θ 2 with respect to the xy-plane, is r 2 = (z 2 -z 3 ) 2 + (y 2 -y 3 ) 2 = 2.29 Å. The angles of the bonds r 1 and r 2 formed with the x and z directions, respectively, are obtained as