The mechanism of the [2+2] cycloaddition reaction of cyclopentyne to ethylene has been studied using the completely renormalized coupled cluster method with singles, doubles, and noniterative triples (CR-CCSD(T)). In agreement with the experimentally observed stereochemistry, the CR-CCSD(T) method favors the concerted pathway involving a [2+1] transition state, whereas the popular CCSD(T) method, which is often regarded as the "gold standard" of electronic structure theory, and low-order multireference methods support the less probable biradical mechanism. In addition, the CCSD(T) approach produces an erroneous description of some transition states and intermediates, particularly those which have a significant biradical character. The CR-CCSD(T) calculations indicate that the reaction is a highly exothermic (Delta G(r)(298) = -68 kcal/mol), predominantly concerted process with a relatively low activation barrier on the order of 13- 16 kcal/mol which permits its thermal occurrence.