Electronic structure of bulk manganese oxide and nickel oxide from coupled cluster theory

We describe the ground- and excited-state electronic structure of bulk MnO and NiO, two prototypical correlated electron materials, using coupled cluster theory with single and double excitations (CCSD). As a corollary, this work also reports an implementation of unrestricted periodic ab initio equation-of-motion CCSD. Starting from a Hartree-Fock reference, we find fundamental gaps of 3.46 and 4.83 eV for MnO and NiO, respectively, for the 16-unit supercell, slightly overestimated compared to experiment, although finite-size scaling suggests that the gap is more severely overestimated in the thermodynamic limit. From the character of the correlated electronic bands we find both MnO and NiO to lie in the intermediate Mott/charge-transfer insulator regime, although NiO appears as a charge transfer insulator when only the fundamental gap is considered. While the lowest quasiparticle excitations are of metal 3d and O 2p character in most of the Brillouin zone, near the Gamma point, the lowest conduction band quasiparticles are of s character. Our study supports the potential of coupled cluster theory to provide high-level many-body insights into correlated solids.