Perdew, J. P.; McMullen, E. R.; Zunger, Alex (1981). "Density-functional theory of the correlation energy in atoms and ions: A simple analytic model and a challenge". Phys. Rev. A. 23 (6): 2785–2789. Bibcode:1981PhRvA..23.2785P. doi:10.1103/PhysRevA.23.2785.
Perdew, J. P.; McMullen, E. R.; Zunger, Alex (1981). "Density-functional theory of the correlation energy in atoms and ions: A simple analytic model and a challenge". Phys. Rev. A. 23 (6): 2785–2789. Bibcode:1981PhRvA..23.2785P. doi:10.1103/PhysRevA.23.2785.