  Standard spin-density functionals for the exchange-correlation energy of a
many-electron ground state make serious self-interaction errors which can be
corrected by the Perdew-Zunger self-interaction correction (SIC). We propose a
size-extensive construction of SIC orbitals which, unlike earlier
constructions, makes SIC computationally efficient, and a true spin-density
functional. The SIC orbitals are constructed from a unitary transformation that
is explicitly dependent on the non-interacting one-particle density matrix.
When this SIC is applied to the local spin-density approximation, improvements
are found for the atomization energies of molecules.
