JAHN-TELLER DISTORTION AND MAGNETIC-STRUCTURES IN LAMNO3

Structural optimization is performed for LaMnO3 with the first-princip les pseudopotential method based on the local density approximation (L DA), the generalized gradient approximation, and the LDA+U approximati on. The Jahn-Teller distortion is reproduced even by LDA but the magni tude of the distortion for all three approximations is not quite large enough compared with the experimental value. The same analysis is per formed also for the hypothetical ferromagnetic state. The Jahn-Teller distortion still exists but the magnitude is much reduced. The unit-ce ll shape becomes nearly cubic. These features are qualitatively consis tent with the related experimental observations. If the crystal struct ure is optimized for each magnetic state, the present band calculation predicts the ferromagnetic state to be lower in energy than the A-typ e antiferromagnetic state, which is inconsistent with the experimental observation. The origin of the failure in the prediction of the groun d state is discussed.