Thanks for your quick reply and sorry for the back and forth. In short, what I would like to do is to *store* only the truncation error (last sweep, center bond) and the corresponding ground-state energy in a file. Ideally, I was thinking that the file would be structured as a two-column table, which I can read in later with Mathematica or some other program.
So, setting Quiet=false does give me the information about the truncation error but does not (if I understand correctly) enable me to write the same to a file in the format I had in mind. If there was a direct way to assign the error to a variable after a dmrg function call, that would be helpful for this purpose.
I will try looking at the DMRGObserver as you suggested to see if it can be modified in any way for this purpose. Thank you once again!