Spin-state energetics of iron(II) porphyrin from the particle-particle random phase approximation Balazs Pinter, Rachael Al-Saadon, Zehua Chen and Weitao Yang Eur. Phys. J. B, 91 11 (2018) 270 Published online: 05 November 2018 DOI: 10.1140/epjb/e2018-90169-6