Usually VASP uses the standard interpolation for the correlation part of the exchange correlation functional. If VOSKOWN is set to 1 the interpolation formula according to Vosko, Wilk and Nusair is used. This usually enhances the magnetic moments and the magnetic energies. Because the Vosko-Wilk-Nusair interpolation is the interpolation usually applied in the context of gradient corrected functionals, it is desirable to use this interpolation whenever the PW91 functional is applied. Setting this tag is not required for the PBE or PBEsol functional, since these functional strictly follow the original publications and disregard this flag entirely (this implicitly implies that the correlation energy is interpolated according to Vosko, Wilk and Nusair).