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 D _4h symmetry: We find three stationary states of 3E _g, 3A _2g, and 3B _2g symmetries of the Fe2 ⁺ ion, and total-energy calculations clearly demonstrate that the ground state is 3A _2g. By contrast, a columnar stacking of the FePc molecules (α-FePc) is found to change the ground state to 3E _g due to hybridization between adjacent molecules.