The controversial ground slate and conformation of the tetramethyleneethane diradical (TME) are discussed from ab initio calculations including electronic correlation. The results for the singlet-tripler energy gap for different conformations of TME calculated with the difference dedicated configuration interaction method (DDCI) are presented. They support the most reliable experimental hypothesis concerning structures and energetics, The equilibrium geometries of both species have been found to be quite different, of D-2d symmetry for the singlet and of D-2 symmetry for the tripler, with dihedral angles between the allyl moieties of 90 degrees and 47 degrees, respectively. Regarding the energies, the singlet always appears to be more stable than the triplet, but the energy difference is so small in the neighborhood of the equilibrium conformation of the tripler, less than 0.3 kcal mol(-1), that both states can be considered degenerate at this geometry. The S-T energy difference is tuned by the torsional coordinate, to the extent that the observed state can be modified when the TME is incorporated ill a larger structure that rotationally constrains the diradical or when the triplet is produced by the reaction mechanism.