Fe(ii)-porphyrins play an important role in many reactions relevant to material science and biological processes, due to their closely lying spin states. Although the prevalent opinion is that these systems posses the triplet ground state, the recent experiment on Fe(ii)-phthalocyanine under conditions matching those of an isolated molecule points toward the quintet ground state. We present a thorough DFT and DMRG-based tailored CC study of Fe(ii)-porphyrin model, in which we address all previously discussed correlation effects. We examine the importance of geometrical parameters, the Fe-N distances in particular, and conclude that the system possesses the quintet ground state.

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic v v i Flemingovo nám 2 16610 Prague 6 Czech Republic and Regional Centre of Advanced Technologies and Materials Palacký University 77146 Olomouc Czech Republic

J Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic v v i Dolejškova 3 18223 Prague 8 Czech Republic

J Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic v v i Dolejškova 3 18223 Prague 8 Czech Republic and Faculty of Mathematics and Physics Charles University Ke Karlovu 3 12116 Prague 2 Czech Republic

Strongly Correlated Systems Lendület Research group Wigner Research Centre for Physics H 1525 Budapest Hungary

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