On relativistic corrections to the S-wave pion-nucleus potential

We analyze possible relativistic corrections to the pion-nucleus optical potential. We find that all relativistic effects which have been under discussion recently are already contained in the standard impulse-approximation result for the optical potential. In particular we show that the corrections due to an in-medium effective mass of the nucleon as recently proposed by Birbrair et al. are simply a result of an inconsistent use of free and in-medium spinors for the nucleons. We observe that in the relativistic framework there are cancellations of very large pieces. Hence usual approximations which are legitimate in most occasions spoil these cancellations here and introduce spurious terms of considerable size. (orig.)