#### pmb

On page 176 problem 6.12 states

"A particle moving in one dimension interacts with a potential V(x). In a stationary state of this system show that

(1/2) <x dV/dx > = <T>

where T = p^2/2m is the kinetic energy of the particle."

Liboff gives the answer but starts off with

"In a stationary state,

d<xp>/dt = (i/hbar)<[H,xp]> = 0

..."

Why? I.e. why is d<xp>/dt = (i/hbar)<[H,xp]> = 0 for a stationary state?

Pete