Ab initio cluster study of the bilayer manganite La2−2xSr1+2xMn2O7

We have performed complete active space self-consistent field (CASSCF) cluster calculations for (MnO6)9− and (MnO6)8− complexes in the background potential appropriate for the bilayer manganite La2−2xSr1+2xMn2O7 at x=0.40. While the ground state of the (MnO6)9− complex is well-described by a restricted Hartree–Fock wavefunction, contributions from configurations with holes in oxygen orbitals are significant in the (MnO6)8− ground state wavefunction. Surprisingly, the CASSCF calculations including sufficient oxygen orbitals in the active space yields a doubly degenerate E (of the C4v point group) as the ground state.