# Semiclassical electron velocity in a periodic potential

 P: 269 Semiclassical electron velocity in a periodic potential is $$v(k)=\langle \psi |p/m| \psi \rangle$$ But if we use the relation $$p/m=\frac{1}{i\hbar} [x,H]$$ we earn v=0, while we know that it should be $$\frac{1}{\hbar} \frac{dE}{dk}$$ Why?