Thanks for the help Would a better, and more consistent, solution potentially be to include a new option for the "open" argument? The option could be named something like "pdbCol21" with the accepted values being "automatic" (default, and the way it works right now), "resname" (always treats column 21 as being a part of the residue name), "chain" (always treats column 21 as being a part of the chain name) and "ignore" (just ignores whatever is in column 21). "resname" and "chain" would of course ignore column 21 if it is blank for a given line. I don't know how much work it would be to implement something like this, but i think it would be helpful as users can then be certain about how their pdb files are read by the program. Kind regards, Mikkel Dahl Andreasen