I am currently learning for an exam and I wonder what the difference between the Lamb shift and the Darwin term is.
What I know is that the Darwin term affects only the s-states of the hydrogen atom and shifts its energy level to that of the p-state, while the Lamb shift affects the p-state and shifts its energy level, so that s- and p-states have different energies after all.
However, what I do not understand is that both are explained in terms of Zitterbewegung. The electron, as it moves around the nucleus, interacts with the electromagnetic field by emitting and absorbing virtual particles and experiences a recoil with each interaction. Thus, its distance from the nucleus varies by a certain $\delta r\approx \lambda_C$, which is accounted for in perturbation theory.
But, in this picture, what is here the actual cause for the Lamb shift and and what is the cause for the Darwin term?