Bergman cyclization of the enediynes (Z)-3-hexene-1,5-diyne (1), (Z)-3-heptene-1,5-diyne (2), (Z)-4-octene-2,6-diyne (3), (Z)-1-cyclononene-3,8-diyne (4), (Z)-1-cyclodecene-3,9-diyne (5), and (Z)-1-cycloundecene-3,10-diyne (6) has been studied by density functional methods. The reaction of 1 was first studied using the BP86, BLYP, BPW91, and B3LYP functionals with the 6-311G** basis set and the large ANO basis set for the latter two functionals. The BPW91/6-311G** calculations yielded results comparing well with those of high-level ab initio computations. Thus, BPW91/6-311G** was employed to study the reactions of 2-6. Geometry optimizations and harmonic frequency calculations were applied for every reactant, transition structure, and product; frequency calculations were also carried out for other optimized stationary points. The optimized structure of the conformer of 6 with the lowest energy agrees excellently with the X-ray diffraction crystallographic structure. IRC (intrinsic reaction coordinate) calculations were carried out for the transition structures of 4, 5, and 6 to establish the reaction path. The zero-point energy collected reaction barriers for 1-6 are 25.16, 27.93, 32.25, 12.09, 20.87, and 76.42 kcal/mol, respectively. Thermodynamic data, Delta H, Delta S, Delta G, Delta H-a, Delta S-a, and Delta G(a), have been evaluated at several temperatures. The temperature effect on the free energy is insignificant. The critical distance, which is the distance between the two carbon atoms forming a new bond, in the transition states of all six reactions is approximately 2.0 Angstrom. The IRC analysis shows that the reaction coordinate is close to the critical distance, and the reactant with a larger critical distance is relatively more stable and has a higher barrier. Therefore, a smaller ring, possessing a larger strain energy and a shorter critical distance, has a lower barrier.