All PBE pseudopotentials were generated using the Fritz-Haber Institute fhi98PP code:
M. Fuchs and M. Scheffler, Comput. Phys. Commun. 119, 67 (1999)
and either employ the Hamann type of norm-conserving pseudopotential,
D.R. Hamann, Phys. Rev. B 40, 2980 (1989),
or the Troullier/Martins type,
N. Troullier and J.L. Martins, Phys. Rev. B 43, 1993 (1991).
For older (now retired) silicon and hydrogen pseudopotentials, P.A. Schultz used D.R Hamann’s new method for generating pseudopotentials (especially developed to handle the pathologies that GGA functionals inject into pseudopotentials) that Don Hamann very generously provided:
D.R. Hamann, unpublished.
Also, alkali atoms, alkaline earths, and several early transition metals atoms and early main group atoms incorporate the partial core correction:
S.G. Louie, S. Froyen, and M.L. Cohen, Phys. Rev. B 26 1738 (1982).
Please check internal documentation of atom files, and cite the appropriate references in any publications.