The structural, electronic, and magnetic properties of mixed-valence compounds are believed to be governed by strong electron correlations. Here we report benchmark density-functional calculations in the spin-polarized generalized-gradient approximation (GGA) for the ground-state properties of doped ${\mathrm{CaMnO}}_3$. We find excellent agreement with all available data, while inclusion of strong correlations in the $\mathrm{GGA}+\mathrm{U}$ scheme impairs this agreement. We demonstrate that formal oxidation states reflect only orbital occupancies, not charge transfer, and resolve outstanding controversies about charge ordering.

• Received 12 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.036402