In quantum mechanics, one of the major concerns is the propagator determination of the system. The propagator is completely expressed in terms of its classical in the Van-Vleck Pauli Formula. In a harmonic oscillator in a time dependent force, the Lagrangian is given by L=1/2m(dx/dt)^2-1/2mw^2x^2-e(t)x (eqn.1) the equation of motion according to Lagrange equation is (d/dt)(dx/dt)+w^2x=-1/m[e(t)] (eqn.2) the general solution (classical path) to this equation consists of a homogeneous plus the particular solution. In order to get the classical action of the system, i need to integrate the Lagrangian (eqn.1) from 0 to T with the classical path now as the x. But it seems that i cannot correctly get the classical action needed for the system. Do i need to substitute the general solution to the Lagrangian or i just have to substitute the homogeneous solution to the simple harmonic oscillator part and then substitute the general solution to the potential term the e(t)x. I'm really bothered regarding with this matter. Can someone enlightened me with this? An answer to this is very much appreciated. Thank you very much. Gibby.