Two-body P-state energies at order 6

We present an analytical calculation of the complete alpha 6 correction to energies of nP levels of two -body systems consisting of the spin -0 or -1/2 extended -size particles with arbitrary masses and magnetic moments. The obtained results apply to a wide class of two -body systems such as hydrogen, positronium, muonium, and pionic or aniprotonic helium ion. We found an additional alpha 6 correction for nP levels of positronium, which was previously overlooked. Our results are also relevant for light muonic atoms, whose accurate theoretical predictions are required for extracting the nuclear charge radii.