Ab initio electronic structure theory for a cluster model of La 2– x Sr x CuO 4

We report the results of ab initio calculations on a model Cu 4 O 12 cluster. The spectrum of the undoped cluster is most appropriately described in a localized valence bond framework. The ground state and low lying excitations can be fit quite well to a Heisenberg Hamiltonian with an anti‐ferromagnetic coupling constant between nearest‐neighbor Cu3 d spins. For a doping level x = 0.25, the electronic states are most easily interpreted in a molecular orbital (Fermi liquid) framework. In addition, doping introduces low‐lying Cu ⟷ O charge transfer excitations. Our best calculations to date associate the ground state with a hole in the Cu3 d anti‐bonding band (Cu 3+ O 2− ), but the state in which the hole is associated with the O2 p band (Cu 2+ O 1− ) is nearly degenerate.