  The electronic structures of the metallic and insulating phases of the alloy
series Ca(2-x)Sr(x)RuO4 (0 <= x <= 2) are calculated using LDA, LDA+U and
Dynamical Mean-Field Approximation methods. In the end members the groundstate
respectively is an orbitally non-degenerate antiferromagnetic insulator (x=0)
and a good metal (x=2). For x>0.5 the observed Curie-Weiss paramagnetic
metallic state which possesses a local moment with the unexpected spin S=1/2,
is explained by the coexistence of localized and itinerant Ru-4d-orbitals. For
0.2<x<0.5 we propose a state with partial orbital and spin ordering. An
effective model for the localized orbital and spin degrees of freedom is
discussed. The metal-insulator transition at x=0.2 is attributed to a switch in
the orbital occupation associated with a structural change of the crystal.
