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

Abstract

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 ${}^{3}{E}_{g}$, ${}^{3}{A}_{2g}$, and ${}^{3}{B}_{2g}$ symmetries of the Fe${}^{2+}$ ion, and total-energy calculations clearly demonstrate that the ground state is ${}^{3}{A}_{2g}$. By contrast, a columnar stacking of the FePc molecules ($\ensuremath{\alpha}$-FePc) is found to change the ground state to ${}^{3}{E}_{g}$ due to hybridization between adjacent molecules.