I’ve been struggling recently to get CDFT jobs to converge in NWCHEM. Rather than continue to play with ~100 heavy-atom structures (of relevance to organic electronic materials) I went to a very simple system of C2H4 / C2F4. This is sometimes studied as a very simple system showing charge transfer.
I had many issues getting this job to run successfully, before realising that I needed a suitably diffuse basis set for the constraints to be applicable!