Hi The group velocity of an electron wavepacket in a homogeneous lattice is vgroup(k) = ∇kEk, where Ek is the dispersion. I have just read an article, where they use this to find the group velocity of a wavepacket in an inhomogeneous lattice, but they use the homogeneous dispersion. I don't quite understand why this is allowed. Can you clarify this? Best, Niles.