Disruption of the $s{p}^2$ bonding by the compression of the $\pi$-electronic orbitals of graphene at various stacking orders

We investigate the behaviour of the $\pi$-electrons under compression and the effect of the stacking order of graphene layers. First we find that electrons can hardly be squeezed through the $s{p}^2$ network, regardless of the stacking order. The largely deformed electronic orbitals (mainly those of $\pi$-electrons) under compression along the $\mathit{\text{c}}$-axis increase interlayer interaction between graphene layers as expected, but surprisingly in a similar way for the A-A and Bernal stacking. On the other hand, the large out-of-plane compression shifts the in-plane phonon frequencies of A-A stacked graphene layers significantly and very differently from Bernal stacked layers. We attribute these results to the $s{p}^2$-electrons filling the low-density central area in a carbon hexagon under compression for the A-A stacking, hence resulting in a non-monotonic change of the $s{p}^2$-bonding. The results strongly suggest not to ignore 3D features of a 2D material.