Two experimental papers seemed to contradict the theoretical predictions that T_e(³B₂) ~ 28 200 cm^-1 and that the cis isomer ³B₂ was more stable than the trans isomer ³B_u. First, a noteworthy experiment by Lisy and Klemperer [J. Chem. Phys. 72, 3880 (1980)] showed that the lowest energy, Auger detectable metastable triplet state of acetylene has no electric dipole moment, implying that it is trans-bent or linear. Second, experiments by Lundberg and Field [J. Chem. Phys. 98, 8384 (1993)] detected six anomalous vibronic feature states in the stimulated emission pumping spectrum of C₂D₂. These peaks were demonstrably downward transitions from the A ¹A₁ state and were most easily assigned as transitions terminating on the cis ³B₂ state with T₀ < 25 820 cm^-1, in contradiction to the 28 200 cm^-1 singlet-triplet gap predicted by Wetmore and Schaefer in 1978. Hence, a new theoretical study was undertaken to (1) see if the singlet-triplet gap might be lowered by 2 500 cm^-1 on going to higher levels of theory, and (2) to verify that the cis isomer remains below the trans isomer on the T₁ surface.

Geometries for the ground state and the lowest-lying cis and trans minima were optimized using configuration interaction with all single and double excitations (CISD) in conjuction with a double-zeta plus polarization (DZP) basis set. At these geometries, more accurate energy differences were computed using coupled-cluster singles and doubles with a perturbative treatment of connected triple substitutions [CCSD(T)] and a triple-zeta plus double-polarization basis with f functions on carbons [TZ(2df,2p)].

These results clearly indicated that the triplet assignment of the SEP spectra was untenable; as unlikely as it might appear, the observed peaks must be transitions to highly vibrationally excited states of the ground electronic state. The observation of strong, isolated features to such highly excited vibrational states would certainly not be expected. In the course of their analysis, Lundberg and Field were also able to explain the earlier Lisy and Klemperer experiments by arguing that the cis triplet is diluted into high vibrational levels of the ground state via spin orbit mixing, which renders the triplet undeflectable and Auger indetectable.

Theoretical results at this level of theory also definitively showed that the cis triplet lies below the trans triplet; this time the energy difference was predicted to be 0.35 eV.