  The reconstruction of the exchange-correlation potential from accurate ab
initio electron densities can provide insights into the limitations of the
currently available approximate functionals and provide guidance for devising
improved approximations for density-functional theory (DFT). For open-shell
systems, the spin density is introduced as an additional fundamental variable
in Spin-DFT. Here, we consider the reconstruction of the corresponding
unrestricted Kohn-Sham potentials from accurate ab initio spin densities. In
particular, we investigate whether it is possible to reconstruct the spin
exchange-correlation potential, which determines the spin density in
spin-unrestricted Kohn-Sham-DFT, despite the numerical difficulties inherent to
the optimization of potentials with finite orbital basis sets. We find that the
recently developed scheme for unambiguously singling out an optimal optimized
potential [J. Chem. Phys. 135, 244102 (2011)] can provide such spin potentials
accurately. This is demonstrated for two test cases, the lithium atom and the
dioxygen molecule, and target (spin) densities from Full-CI and CASSCF
calculations, respectively.
