  The electronic structure of the perovskite LaCoO$_3$ for different spin
states of Co ions was calculated in the LDA+U approach. The ground state was
found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat
higher in energy we found two intermediate-spin states followed by a high-spin
state at significantly higher energy. The calculation results show that Co 3$d$
states of $t_{2g}$ symmetry form narrow bands which could easily localize
whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$
states, form a broad $\sigma^*$ band. With the increase of temperature which is
simulated by the corresponding increase of the lattice parameter, the
transition from the low- to intermediate-spin states occurs. This
intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering
which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K.
Possible explanations of the magnetic behavior and gradual insulating-metal
transition are suggested.
