Someone please tell me if I am thinking right: Let's consider an unperturbed electronic state of an atom/molecule. If we denote it by [a>, then the average electronic momentum in state [a> is, <p> = <a]p[a> = (<a]p<a])* (because p is hermitian) = (<a]*p*[a>*) = (<a]p*[a>) (because [a> is real.) = - (<a]p[a>) ( because p is purely imaginary. so p* = -p) so, <p> = 0 so, the average momentum in any unperturbed eigenstate = 0 Now, the atom/molecule is being perturbed by a time-dependent scalar potential of the form V(t) = V Cos(wt). So we can write the time-dependent wave function as a perturbation series. Since there is no magnetic field, the wave function will be real and <p> = 0. Is it true? Please correct me if I am doing it wrong. Thanks.