  We use a Heisenberg spin-1/2 chain to investigate how chaos and localization
may affect the entanglement of pairs of qubits. To measure how much entangled a
pair is, we compute its concurrence, which is then analyzed in the
delocalized/localized and in the chaotic/non-chaotic regimes. Our results
indicate that chaos reduces entanglement and that entanglement decreases in the
region of strong localization. In the transition region from a chaotic to a
non-chaotic regime localization increases entanglement. We also show that
entanglement is larger for strongly interacting qubits (nearest neighbors) than
for weakly interacting qubits (next and next-next neighbors).
