Limitations of current density functional theories for the description of partial π-bond breaking

It is shown on the example of the torsional potential of butadiene, that several density functionals including gradient corrected non-local functionals exhibit an intrinsic error that leads to the overestimation of rotational barriers due to partial π-bond breaking. This problem occurs for all functionals investigated including SVWN, BLYP, B3LYP, BPL and BP86 and is independent of the basis set. The hybrid exchange functional (B3) is slightly better than the others. These errors are absent for torsional potentials around single bonds, such as in 1-butene and indicates that the problem may be related to the overestimation of conjugation energy.