A comparison between DFT and other ab initio schemes on the activation energy in the automerization of cyclobutadiene

The influence of the multireference character in the transition state for the automerization reaction of cyclobutadiene is considered. We have analyzed two forms of taking into account this effect: either by the use of two-body density functionals or traditional density functional theory (DFT) correlation functionals conveniently modified. Comparison has also been made with conventional density functional theory Kohn–Sham (DFT–KS) [15]approaches. It is shown that only when the aforementioned multideterminantal character is included in the computational scheme is the activation energy in accord with accurate benchmark calculations.