REKS-type (Filatov, M.; Shaik, S. Chem. Phys. Lett. 1999, 304, 429) density functional calculations were carried out for the lowest energy singlet and triplet states of tetramethyleneethane (TME) diradical. The calculations indicate that the ground state of TME in the gas phase is the singlet state, whereas the triplet state should be metastable at low temperatures. The triplet state metastability derives from the energetic preference for the triplet state at the optimal triplet molecular geometry and from the extremely small (<0.05 cm(-1)) spin-orbit coupling between the triplet and ground singlet states. REKS calculations predict that, in the vibrational spectra of the two states of TME, the modes corresponding to symmetric scissoring vibrations should have nearly the same frequencies of 357 cm(-1), in accord with the experimental observation of two modes of the same frequency of 335 cm(-1) in the vibrational structure of the photoelectron spectrum of the [TME](-) anion.