By definition, X nm only has the frequency (E n − E m)/ h, so its time evolution is simple: X n m ( t ) = e 2 π π i ( E n − − E m ) t / h X n m ( 0 ) = e i ( E n − − E m ) t / ℏ ℏ X n m ( 0 ) . This is the original form of Heisenberg's equation of motion.