It would be really appreciated if somebody could clarify something for me: I know that stationary states are states of definite energy. But are all states of definite energy also stationary state? This question occurred to me when I considered the free particle(plane wave, not a Gaussian packet). The time derivative of the mean value of the position is not constant in time but equal to hbar*k/m. But, plane waves are energy(and momentum) eigenstates for the free particle, that means that they are states of definite energy. So, what's the deal here? Thanks!