  We study the Hartree model for two electrons with spin, living in the
two-dimensional or three-dimensional space with Coulomb interactions and
submitted to the potential induced by two nuclei of charge +1. In the limit
where the nuclei move away from each other, we show that the two lowest
eigenfunctions of the mean-field hamiltonian are asymptotically given by an
even, respectively odd, superposition of the minimizer associated with the
corresponding Hartree one nucleus model. We then give upper and lower bounds on
the exponentially small gap between the first two eigenvalues, due to
(nonlinear) quantum tunneling.
