  We present a unitary correlation operator that explicitly induces into shell
model type many-body states short ranged two-body correlations caused by the
strong repulsive core and the pronounced tensor part of the nucleon-nucleon
interaction. Alternatively an effective Hamiltonian can be defined by applying
this unitary correlator to the realistic nucleon-nucleon interaction.
  The momentum space representation shows that realistic interactions which
differ in their short range behaviour are mapped on the same correlated
Hamiltonian, indicating a successful provision for the correlations at high
momenta. Calculations for He4 using the one- and two-body part of the
correlated Hamiltonian compare favorably with exact many-body methods. For
heavier nuclei like O16 and Ca40 where exact many-body calculations are not
possible we compare our results with other approximations. The correlated
single-particle momentum distributions describe the occupation of states above
the Fermi momentum. The Unitary Correlation Operator Method (UCOM) can be used
in mean-field and shell model configuration spaces that are not able to
describe these repulsive and tensor correlations explicitly.
