The energetics of the Bergman cyclization of (Z)-hexa-1,5-diyn-3-ene (1) to 1,4-didehydrobenzene (2) have been investigated at the CCSD(T)/6-31G(d,p) level of theory, optimizing various structures along the reaction path. The calculated reaction energy and classical barrier are 5.5 and 29.5 kcal/mol, respectively. Vibrational and temperature corrections lead to a reaction enthalpy, Delta(R)H(298), of 8.0 kcal/mol and an activation enthalpy, Delta H-double dagger(298), of 28.5 kcal/mol, indicating that previous determinations of these energies are seriously in error. Although the transition state of the reaction is located in the exit channel, it shows no biradical character, which suggests that the electronic structure of 1 is retained as long as possible. Energy changes in the entrance channel of the reaction are influenced by changes in strain caused by trans bending of the two triple bonds of 1. If the terminal C atoms are pulled together to a distance of 3.0 Angstrom, the reaction becomes spontaneous (Delta H-double dagger (298) = 19.3 kcal/mol) and slightly exothermic (Delta(R)H(298) = -0.8 kcal/mol) due to an increase of bending strain by 8.8 kcal/mol. For 1,2-didehydrobenzene (11), 1,3-didehydrobenzene (12), and 1,4-didehydrobenzene (2), relative energies are calculated to be 0, 13.6, and 25.4 kcal/mol, respectively, while the corresponding Delta H-f degrees(298) values are 108.7, 122.8, and 134.4 kcal/mol, respectively, at the CCSD(T)/6-31G(d,p) level of theory.