  Fe(II)-porphyrins (FeP) play an important role in many reactions relevant to
material science and biological processes, due to their closely lying spin
states. However, this small energetic separation also makes it challenging to
establish the correct spin state ordering. 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 study of FeP
model by means of the density functional theory and density matrix
renormalization group based tailored coupled clusters, 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, which is in our calculations
well-separated from the triplet state.
