  We propose a multiconfigurational hybrid density-functional theory which
rigorously combines a multiconfiguration self-consistent-field calculation with
a density-functional approximation based on a linear decomposition of the
electron-electron interaction. This gives a straightforward extension of the
usual hybrid approximations by essentially adding a fraction \lambda of exact
static correlation in addition to the fraction \lambda of exact exchange. Test
calculations on the cycloaddition reactions of ozone with ethylene or acetylene
and the dissociation of diatomic molecules with the Perdew-Burke-Ernzerhof
(PBE) and Becke-Lee-Yang-Parr (BLYP) density functionals show that a good value
of \lambda is 0.25, as in the usual hybrid approximations. The results suggest
that the proposed multiconfigurational hybrid approximations can improve over
usual density-functional calculations for situations with strong static
correlation effects.
