  Fits of pion-nucleus potentials to large sets of pionic atom data reveal
departures of parameter values from the corresponding free $\pi N$ parameters.
These medium effects can be quantitatively reproduced by a chiral-motivated
model where the pion decay constant is modified in the medium or by including
the empirical on-shell energy dependence of the amplitudes. No consistency is
obtained between pionic atoms and the free $\pi N$ interaction when an extreme
off-shell chiral model is used. The role of the size of data sets is briefly
discussed.

Nuclear Theory