  Although iron-based catalysts are regarded as a promising alternative to
precious metal catalysts, their precise electronic structures during catalysis
still pose challenges for computational descriptions. A particularly urgent
question is the influence of the environment on the electronic structure, and
how to describe this properly with computational methods. Here, we study an
iron porphyrin chloride complex adsorbed on a graphene sheet using density
functional theory calculations to detail how much the electronic structure is
influenced by the presence of a graphene layer. Our results indicate that weak
interactions due to van der Waals forces dominate between the porphyrin complex
and graphene, and only a small amount of charge is transferred between the two
entities. Furthermore, the interplay of the ligand field environment, strong
$p$ $-$ $d$ hybridization, and correlation effects within the complex are
strongly involved in determining the spin state of the iron ion. By bridging
molecular chemistry and solid state physics, this study provides first steps
towards a joint analysis of the properties of iron-based catalysts from first
principles.
