  A new class of orbital-dependent exchange-correlation (xc) potentials for
applications in noncollinear spin-density-functional theory is developed.
Starting from the optimized effective potential (OEP) formalism for the exact
exchange potential - generalized to the noncollinear case - correlation effects
are added via a self-consistent procedure inspired by the
Singwi-Tosi-Land-Sjolander (STLS) method. The orbital-dependent xc potentials
are applied to the Hubbard dimer in uniform and noncollinear magnetic fields
and compared to exact diagonalization and to the Bethe-ansatz local
spin-density approximation. The STLS gives the overall best performance for
total energies, densities and magnetizations, particularly in the weakly to
moderately correlated regime.
