Hello all, I have a fundamental problem with the concept of the propagator regarding to time dependent potential. If the propagator is based on the expansion of the time evolution operator, how can it act on initial wave function in the presence of time dependent potential? You cannot use the evolution operator with these kind of potentials. I can accept the solution of Feynman path integrals, because in this frame, the time is split to many short intervals in which the time evolution operator is valid. However, can you still calculate the propagator by direct calculation? Thanks, The Samurai.