It is known that whenever the Hamiltonian contains an interaction part, the two interacting subsystems are entangled, thus an electron is surely entangled to a proton in a hydrogen atom. The discrepancy in mass of the two particles leads to a rather small entanglement, but it seems that no one has given a quantitative measure for it.  provides analysis on the reduced density function of the system, but gives no quantification;  measures entanglement in hyperfine interaction and the thermal entanglement with concurrence, but the entanglement in orbital motion is not defined. I have seen such entanglement quantified with von Neumann entropy for the internal motion of helium, where the atom is partitioned into two interacting electron-proton subsystems. So, is there a way to regard the electron and the proton as individual subsystems and quantify their orbital entanglement with one another?