 This is still a harder problem than classical mechanics, though, because instead of 
propagating single positions of particles, we must propagate entire functions.In classical mechanics, we could simulate the
motion of the particles by setting formula_23, if we assume that the force is constant over the time span of formula_24.For the imaginary time Schrödinger equation, instead, we propagate forward in time using a convolution integral with a special function called a Green's function.