Hartree-Fock and limited configuration interaction calculations were used to investigate the 5Σ+ and related states of FeO with equilibrium separation of [inverted lazy s] 3.2 bohr. Very extended configuration interaction calculations were then performed to determine the properties of the lowest 5Σ+ state. This lowest state is determined to be close to the Hartree-Fock state investigated. The dissociation energy is estimated to be 2.00 ± 0.14 eV. It is concluded that the ground state of FeO is not 5Σ+. © 1973, The American Institute of Physics. All rights reserved.