Orbital ordering and volume collapse in cerium*

Realistic all-orbital many-body calculations are performed for cerium, covering a large volume range. $16$-orbital cerium model Hamiltonians are obtained from the local-density approximation and solved using the fluctuation-exchange approximation. It is found that the thermodynamic behavior of cerium is coupled to orbital-ordering transitions of its $f$ electrons. Different arrangements of $f$ electrons occur for the $\alpha$- and $\gamma$-cerium; moreover, metastable solutions also exist. In addition magnetic and spectral properties are analyzed.