In the undeformed lattice, the length of the auxillary bonds OC and OD as shown in the upper panel of Fig. is (r 1 sin θ 1 + 2r 2 cos θ 2 ) 2 + (r 1 cos θ 1 ) 2 = 3.45 Å, which is 1.53 times the bond length r 1 = r OA = r OB = 2.25 Å. Before the structural transition at ϵ y = 24.79%, r OA = The reduction in the second-neighbor bond lengths facilitate attraction. We define critical length (r fm ) as the bond length between the nearest-neighbor pairs at which the bond force is the maximum, where bond fracture and formation can occur. If the second-neighbor bond length, which is initially much higher than the r fm in the undeformed state, grows close to r fm before the first-neighbor bond reaches r fm , the lattice favorably experiences a bond formation between half of the secondneighbor atoms at the cost of breaking half of the firstneighbor bonds. Thus, for the structural phase transition to happen, the mathematical criteria of r OD → r fm must be met. At the macroscopic or applied strain ϵ y , the length of the OA bond becomes λ ′ 1 = λ 1 (1 + ϵ y ), while the length of the OB bond is λ ′ 2 = λ 2 (1 -νϵ y ). The condition, under the assumption of linear deformation, can thus be written as