A theoretical evaluation of tetra-tert-butylethylene (1) at the BLYP/DZd level confirms that it should be a stable molecule with a singlet ground state. The synthesis of 1 from two molecules of di-tert-butylcarbene (6) is unlikely. Although the formation of singlet 1 from the triplet B-3 ground state of 6 (singlet 6 is only 1-3 kcal/mol higher in energy) is highly exothermic (Delta H = -73.7 kcal/mol), the barrier Delta G(double dagger) = 25 kcal/mol (298 K, 1 atm, BLYP/DZd) for the dimerization is too large to compete with the barrier for intramolecular carbene insertion. The barrier for singlet 6 to yield 1,1-dimethyl-2-tert-butylcyclopropane (12) is only 5 kcal/mol. The CC double bond in singlet 1 is twisted by 45 degrees, and the strain energy is similar to 93 kcal/mol in agreement with molecular mechanics results. Triplet 1 has a nearly perfectly perpendicular conformation at the central CC bond (87 degrees torsional angle), but it is still strained by 42 kcal/mol and is 12 kcal/mol higher in energy than singlet 1. Alkyl substitution decreases the S-T separation of carbenes due to the greater hyperconjugative stabilization of the singlet than the triplet.