  For the one-dimensional Hubbard model subject to periodic boundary conditions
we construct a unitary transformation between basis states so that open
boundary conditions apply for the transformed Hamiltonian. Despite the fact
that the one-particle and two-particle interaction matrices link nearest and
next-nearest neighbors only, the performance of the density-matrix
renormalization group method for the transformed Hamiltonian does not improve.
Some of the new interactions act as independent quantum channels which generate
the same level of entanglement as periodic boundary conditions in the original
formulation of the Hubbard model. We provide a detailed analysis of these
channels and show that, apart from locality of the interactions, the
performance of DMRG is effected significantly by the number and the strength of
the quantum channels which entangle the DMRG blocks.

Strongly Correlated Electrons