  We apply the analytically solvable model of two electrons in two orbitals to
diradical molecules, characterized by two unpaired electrons. The effect of the
doubly occupied and empty orbitals is taken into account by means of random
phase approximation (RPA). We show that in the static limit the direct RPA
leads to the renormalization of the parameters of the two-orbital model. We
test our model by comparing its predictions for the singlet-triplet splitting
with the results from multi-reference CASSCF and NEVPT2 simulations for a set
of ten molecules. We find that, for the whole set, the average relative
difference between the singlet-triplet gaps predicted by the RPA-corrected
two-orbital model and by NEVPT2 is about 40%. For the five molecules with the
smallest singlet-triplet splitting the accuracy is better than 20%.
