Low-lying energy spectrum of the cerium dimer

The electronic structure of Ce2 is studied in a valence bond model with two 4f electrons localized at two cerium sites. It is shown that the low-lying energy spectrum of the simplest cerium chemical bond is determined by peculiarities of the occupied 4f states. The model allows for an analytical solution which is discussed along with the numerical analysis. The energy spectrum is a result of the interplay between the 4f valence bond exchange, the 4f Coulomb repulsion and the spin-orbit coupling. The calculated ground state is the even \Omega=\Lambda=\Sigma=0 level, the lowest excitations situated at 30 K are the odd \Omega=\Lambda=\Sigma=0 state and the $^{3} 6_5$ doublet (\Omega=\pm5,\, \Lambda=\pm6,\, \Sigma=\mp1). The calculated magnetic susceptibility displays different behavior at high and low temperatures. In the absence of the spin-orbit coupling the ground state is the ${}^3 \Sigma_g^-$ triplet. The results are compared with other many-electron calculations and experimental data.