  We evaluate the contribution of second order terms to the pion-nucleus s-wave
optical potential of pionic atoms generated by short range nuclear correlation.
The corrections are sizeable because they involve the isoscalar s-wave $\pi N$
amplitude for half off-shell situations where the amplitude is considerably
larger than the on-shell one. In addition, the s-wave optical potential is
reanalyzed by looking at all the different conventional contributions together
lowest order, Pauli corrected rescattering term, second order absorptive
effects, terms from the interaction of pions with the virtual pion cloud
(chiral corrections) and correlation effects. Different off-shell
extrapolations for the $\pi N$ amplitude are used and it is found that,
although some individual terms are sensitive to the extrapolation, the sum of
them is rather insensitive. The results are compared with empirical values from
best fits to the data and are found to be compatible, within theoretical and
empirical uncertainties. The results do not rule out further contributions but
they put stringent constraints on their strength.
