  We extend the post-processing finite-size (FS) correction method, developed
by Kwee, Zhang, and Krakauer [Phys. Rev. Lett. 100, 126404 (2008)], to spin
polarized systems. The method estimates the FS effects in many-body electronic
structure calculations of extended systems by a modified density functional
theory (DFT) calculation, without having to repeat expensive many-body
simulations. We construct a unified FS DFT exchange-correlation functional for
spin unpolarized and fully spin polarized systems, under the local density
approximation. The results are then interpolated to arbitrary spin
polarizations. Generalization to other functional forms in DFT are discussed.
The application of this FS correction method to several typical magnetic
systems with varying supercell sizes demonstrates that it consistently removes
most of the FS errors, leading to rapid convergence of the many-body results to
the infinite size limit.
