In extended systems, it is often impractical to repeat calculations with different functional approximations [see contribution (2.1.16)], and it is essential to develop a level of "trust" [see contribution (3.2.7)] in the approximations one uses and a feeling for their limitations. My own applications over several decades show a clear preference for main-group elements, which might imply less trust in the ability of particular DFAs to describe transition and rare-earth elements. This is perhaps not surprising, since some DFAs describe energy differences in the corresponding atoms very poorly.