Bond stretching branch dispersions in undoped La₂CuO₄ calculated using LDA+U

In undoped, insulating La₂CuO₄ (LCO), the longitudinal Cu-O bond-stretching branches are observed to be nearly dispersionless. As doping increases, these branches soften away from the zone center and disperse downwards towards the zone boundary. LDA calculations, which predict the undoped cuprates to be metallic, do not find the nearly dispersionless character of these modes. In the LDA, these branches disperse downward towards the zone boundary as in overdoped metallic LCO. In this work, we employ the Hubbard-U correction in the LDA+U method to calculate the dispersions for a range of U values. With U=6 eV, we find the insulating, antiferromagnetic ground state of LCO. Our calculations predict the bond-stretching modes to be nearly dispersionless in the insulating ground state in agreement with experiment.