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Fluctuation-exchange approximation theory of the non-equilibrium singlet-triplet transition

As a continuation of a previous work [B. Horváth et al., Phys. Rev. B {\bf 82}, 165129 (2010)], here we extend the so-called Fluctuation Exchange Approximation (FLEX) to study the non-equilibrium singlet-triplet transition. We show that, while being relatively fast and a conserving approximation, FLEX is able to recover all important features of the transition, including the evolution of the linear conductance throughout the transition, the two-stage Kondo effect on the triplet side, and the gradual opening of the singlet-triplet gap on the triplet side of the transition. A comparison with numerical renormalization group calculations also shows that FLEX captures rather well the width of the Kondo resonance. FLEX thus offers a viable route to describe correlated multi-level systems under non-equilibrium conditions, and, in its rather general form, as formulated here, it could find a broad application in molecular electronics calculations.