The autoaromatization of (Z)-hex-3-ene-1,5-diyne to the singlet biradical p-benzyne has been reinvestigated by state of the art ab initio methods. Previous CCSD(T)/6-31G(d,p) and CASPT2[0]/ANO[C(5s4p2dlf)/H(3s2p)] calculations estimated the reaction heat at 298 K to be 8-10 and 3.9 +/- 3.2 kcal/mol, respectively. Recent NO- and oxygen-dependent trapping experiments and collision-induced dissociation threshold energy experiments estimate the heat of reaction to be 8.5 +/- 1.0 kcal/mol at 470 K (corrected to 9.5 +/- 1.0 kcal/mol at 298 K) and 8.4 +/- 3.0 kcal/mol at 298 K, respectively. New theoretical estimates at 298 K predict the values at the basis set Limit for the CCSD(T) and CASPT2[gl] methods to be 12.7 +/- 2.0 and 5.4 +/- 2.0 kcal/mol, respectively. The experimentally predicted electronic contribution to the heat of activation is 28.6 kcal/mol. This can be compared with 25.5 and 29.8 kcal/mol from the CASPT2[gl] and the CCSD(T) methods, respectively. The new study has a much larger one-particle basis set for the CCSD(T) method as compared to earlier studies. For the CASPT2 investigation the better suited CASPT2[gl] approximation is utilized. The original CASPT2 method, CASPT2[0], systematically favors open-shell systems relative to closed-shell systems. This was previously corrected empirically.