  The role of shake-off for double ionization of atoms by a single photon with
finite energy has become the subject of debate. In this letter, we attempt to
clarify the meaning of shake-off at low photon energies by comparing different
formulations appearing in the literature and by suggesting a working
definition. Moreover, we elaborate on the foundation and justification of a
mixed quantum-classical ansatz for the calculation of single-photon double
ionization.
