Potential energy curves and generalized oscillator strength for doubly excited states of hydrogen molecule

We report calculations of potential energy curves for doubly excited states of the H₂ molecule in the $1.2a.u.⩽R⩽12a.u.$ range at the multireference configuration interaction (MRCI) level based on multiconfigurational self-consistent field (MCSCF) orbitals. We focus on the Q₂ states which lie between the second and third ionization thresholds of the $H_2^{+}$ molecular ion, i.e. the ${}^2{\Sigma }_u^{+}$ and ${}^2{\Pi }_u$ states, respectively, and dissociate in the H(2l) + H(2l) channels. The MCSCF/MRCI approach allows us to successfully identify the molecular optically forbidden $Q_2{}^1{\Sigma }_g^{+}$ state which dissociates into two H(2s) atoms. Generalized oscillator strength as a function of transferred momentum for three doubly excited states is also presented. (paper)