> I am trying to freeze atom ID numbers: 18-54-17-55
> 54, 55 are the newly added atoms.
Ok, this is definitely a bug. Sorry to take a while to get back to you, but it was hard to figure out what was wrong. There are a few issues:
1) The atom indexes in the constraints box are numbered from 0. So you'd want 17-53-16-54.
2) The up-and-down arrows seem limited, but you can manually type numbers into the boxes.
So I entered the constraint above (17-53-16-54) and was able to enforce a constraint.
I'm going to fix both of these bugs for 1.1.
Thanks very much,