Both of these features (and more) are incorporated in the relativistic Dirac equation, with predictions that come still closer to experiment. Again the Dirac equation may be solved analytically in the special case of a two-body system, such as the hydrogen atom. The resulting solution quantum states now must be classified by the total angular momentum number (arising through the coupling between electron spin and orbital angular momentum). States of the same and the same are still degenerate. Thus, direct analytical solution of Dirac equation predicts 2S() and 2P() levels of hydrogen to have exactly the same energy, which is in a contradiction with observations (Lamb–Retherford experiment).