Abstract
We present a theoretical study of the spin state of LaCoO3 using local density approximation + Hubbard U (LDA+U) approach based on pseudopotential method. The correlation effects play a significant role in the case of low spin (LS), intermediate spin (IS) and high spin (HS) configurations. The ground state is found to be a nonmagnetic insulator with Co ions in a LS state. Somewhat higher in energy, we find an IS state followed by a HS state at significantly higher energy. The orbital ordering in IS state driven by the Jahn-Teller (JT) and on-site Coulomb interactions is demonstrated based on the LDA+U electronic structures. The phonon dispersion curve of the paramagnetic LS ground state at high symmetry directions of the Brillouin zone (BZ) and phonon density of states are also calculated. The frequencies of the Raman and IR modes are calculated and the main characteristics of these modes are discussed. The computed zone center phonon frequencies of LS state agree fairly well with the experimental results.