Constraint density functional calculations for multiplets in a ligand-field applied to Fe-phthalocyanine

Multiplets in a ligand field are treated within total-energy density-functional calculations by imposing density-matrix constraints on the d-orbital occupation numbers consistent with the local site and state symmetries. We demonstrate the utility of this approach for the case of isolated Fe phthalocyanine (FePc) molecules with overall D₄ₕ symmetry: We find three stationary states of ³Eg, ³A2g, and ³B2g symmetries of the Fe²⁺ ion, and total-energy calculations clearly demonstrate that the ground state is ³A2g. By contrast, a columnar stacking of the FePc molecules (α-FePc) is found to change the ground state to ³Eg due to hybridization between adjacent molecules.