  In-medium subthreshold KbarN scattering amplitudes calculated within a
chirally motivated meson-baryon coupled-channel model are used self
consistently to confront K^- atom data across the periodic table. Substantially
deeper K^- nuclear potentials are obtained compared to the shallow potentials
derived in some approaches from threshold amplitudes, with Re V_{chiral} =
-(85+/-5) MeV at nuclear matter density. When KbarNN contributions are
incorporated phenomenologically, a very deep K^- nuclear potential results, Re
V_{chiral+phen.} = -(180+/-5) MeV, in agreement with density dependent
potentials obtained in purely phenomenological fits to the data. Self
consistent dynamical calculations of K^- nuclear quasibound states are reported
and discussed.
