MP2 is a post-SCF ground-state correlation correction: the SCF reference
(RHF/UHF/ROHF) is converged first by the usual PyOQP reference step, and
this driver adds the second-order Moller-Plesset correlation energy on top,
reusing the validated two-electron driver via mp2_lib. It is dispatched
from Python as [input] method = mp2 (a ground-state post-SCF method that
reports no excitations).