Spin-state energetics of iron(II) porphyrin from the particle-particle random phase approximationBalazs Pinter, Rachael Al-Saadon, Zehua Chen and Weitao YangEur. Phys. J. B, 91 11 (2018) 270DOI: https://doi.org/10.1140/epjb/e2018-90169-6