  Stacking graphene sheets forms graphite. Two in-plane vibrational modes of
graphite, E1u and E2g(2), are derived from graphene E2g mode, the shifts of
which under compression are all considered as results of in-plane bond
shortening. Values of Gruneisen parameter have been reported to quantify such
relation. However, the reason why the shift rates of these three modes with
pressure differ is unclear. In this work, we introduce a new parameter to
quantify the contribution of out-of-plane strain to the in-plane vibrational
frequencies, suggesting that the compression of \pi-electrons plays a
non-negligible part in both graphite and graphene under high pressure.
