(a) Top view of α phase group-V elemental ML. The unit cell is indicated with black dashed rectangle lines. (b) Free energy contour for α-As ML versus the buckling heights (h_U, h_L). The phases A, A′, and B are marked. (c) Side views of the two energy-degenerate distorted noncentrosymmetric structures (A and A′ phases) and undistorted centrosymmetric structure (phase B). The height differences between upper (red) and lower (blue) atomic layers are marked as h_U and h_L, respectively. (d) Side view of α-Bi ML in antiferroelectric (AFE) configuration. The atomic sites shifted up and down are colored in red and green, respectively. (e) Free energy contour of Bi MLs versus buckling heights (h^(R), h^(G)). For Bi, besides the FE phases A and A′, there also exist metastable AFE phases C and C′.^[43]