{\it Ab-initio} studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO3 and KCuF3. We present a new Average Spin State (ASS) calculational scheme that allowed us to treat a paramagnetic state. Using this scheme, we succesfully described the experimental magnetic/orbital phase diagram of both LaMnO3 and KCuF3 in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.
@article{arxiv.cond-mat/0112423,
title = {Orbital Ordering in Paramagnetic LaMnO3 and KCuF3},
author = {J. E. Medvedeva and M. A. Korotin and V. I. Anisimov and A. J. Freeman},
journal= {arXiv preprint arXiv:cond-mat/0112423},
year = {2009}
}