The vibronic coupling constants of C − 60 are derived from the photoelectron spectrum measured by Wang et al. [X. B. Wang, H. K. Woo, and L. S. Wang, J. Chem. Phys., 123, 051106 (2005).] at low temperature with high-resolutions. We find that the couplings of the Jahn-Teller modes of C − 60 are weaker than the couplings reported by Gunnarsson et al. [O.

The dynamical Jahn-Teller effect on fullerene sites in insulating Cs(3)C(60) is investigated fully ab initio. The vibronic excitations of rotational type are at ≥ 65 cm(-1), while the net kinetic contribution to the Jahn-Teller stabilization energy constitutes approximately 90 meV. This means that no localization of distortions by intermolecular interactions is possible in these fullerides; therefore, free rotations of deformations take place independently on each C(60). The latter destroy the orbital ordering and establish a conventional exchange interaction between S = 1/2 on fullerene sites. The corresponding exchange model is derived and predicts the Néel temperature for A15 Cs(3)C(60) close to experiment.

Analytical expressions for the exchange interaction between J-multiplets of interacting metallic centers are derived on the basis of a complete electronic model which includes the intrasite relativistic effects. A common belief that this interaction can be approximated by an isotropic form ∝ J1 · J2 (or ∝ J1 · S2 in the case of interaction with an isotropic spin) is found to be ungrounded. It is also shown that the often used "1/U approximation" for the description of the kinetic contribution of the exchange interaction is not valid in the case of J-multiplets. The developed theory can be used for microscopic description of exchange interaction in materials containing lanthanides, actinides and some transition metal ions.

Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed (Ln = Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

Dynamical Jahn-Teller effect has escaped so far direct observation in metallic systems. It is particularly believed to be quenched also in correlated conductors with orbitally degenerate sites such as cubic fullerides. Here the Gutzwiller approach is extended to treat electron correlation over metals with Jahn-Teller active sites and applied to the investigation of the ground state of K3C60. It is shown that dynamical Jahn-Teller instability fully develops in this material when the interelectron repulsion U on C60 sites exceeds some critical value. The latter is found to be lower than the current estimates of U , meaning that dynamical Jahn-Teller effect takes place in all cubic fullerides. This leads to strong splitting of LUMO orbitals on C60 sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in fullerides.