---
upgrade:
  - |
    Set default ``fusion_max_qubit`` and ``fusion_threshold`` depending on the configured
    ``method`` for :class:`~AerSimulator`. Previously, the default values of
    ``fusion_max_qubit`` and ``fusion_threshold`` were ``5`` and ``14`` respectively for
    all simulation methods. However, their optimal values depend on running methods. If you
    depended on the previous defaults you can explicitly set ``fusion_max_qubit=5`` or
    ``fusion_threshold=14`` to retain the previous default behavior. For example::

        from qiskit_aer import AerSimulator

        sim = AerSimulator(method='mps', fusion_max_qubit=5, fusion_threshold=14)
