* numbers in brackets refer to corresponding numbers in the publication list
Most of the results were obtained in collaboration with PHD students, guests and colleagues. They often contributed most of the work.
Development of a theory for the description of crystal-field excitation and their interactions with electrons and phonons.
Interactions with electrons
A theory of the anormalous thermopower in a system with crystal-field split ions was derived (with I. Peschel, H. Takayama) and experimentally verified (with J. Sierro, E. Umlauf).
An explanation of the strong magnetic field dependence of the effective mass in Pr metal was given (with R. White and J. Jensen). [80, 89]
A theory of the line width of crystal-field excitations was worked out (with K. Becker, J. Keller). 
Interactions with phonons
A microscopic derivation of the magnetoeleastic interactions in rare-earth systems was given (with V. Dohm). Predictions on the importance of rotational interactions were verified experimentally by B. Lüthi. 
A prediction of a Faraday rotation of phonons and of a phononic Cotton-Mouton effect in rare-earth systems was made (with P. Thalmeier). [63, 68] These effects were verified experimentally.
The existence of bound states between phonons and crystal-field excitations was shown (with P. Thalmeier). This explained neutron scattering experiments by M. Loewenhaupt on CeAl2 and later experiments on certain cuprates. 
Theoretical models for superionic conductors were developed with W. Dietrich, I. Peschel and S. Strässler et al. were developed. [52, 76]
Two lines of research were pursued. One consists in determining many-electron wavefunctions for the ground state of solids. The achieved accuracy is comparable with the one obtained for small molecules when quantum chemical methods are applied. For that purpose a theoretical framework had to be created and applied.
The second line of research dealt with strongly correlated electrons and is of more phenomenological nature. Different origins of heavy fermion behavior were identified and methods were developed to calculate the excitation spectrum of strongly correlated electrons.
A substantial part of that research has been described in a book on “Electron Correlations in Molecules and Solids” (3rd edition, Springer Verlag 1995).