  We apply the density-matrix renormalization group technique to magnetic
molecules in order to evaluate the low-lying energy spectrum. In particular, we
investigate the giant Keplerate molecule Mo72Fe30 where 30 Fe3+ ions (spins
5/2) occupy the sites of an icosidodecahedron and interact via nearest-neighbor
antiferromagnetic Heisenberg exchange.
  The aim of our investigation is to verify the applicability and feasibility
of DMRG calculations for complex magnetic molecules. To this end we first use a
fictitious molecule with the same structure as Mo72Fe30 but with spins 1/2 as a
test system. Here we investigate the accuracy of our DMRG implementation in
comparison to numerically exact results. Then we apply the algorithm to
Mo72Fe30 and calculate an approximation of the lowest energy levels in the
subspaces of total magnetic quantum number. The results prove the existence of
a lowest rotational band, which was predicted recently.
