Using fix gcmc with fix rigid/small langevin

Dear Developers and Users,
I would like to incorporate the grand canonical Monte Carlo method in order to maintain a fixed chemical potential in a system consisting of small rigid bodies influenced by langevin dynamics. To demonstrate what I am trying to accomplish, I am attaching an input file which constitutes a simple diatomic system. This particular system is able to exchange molecules, but once molecules have been inserted into the box the relevant dynamics do not appear to take place. I would appreciate insight into what I can do to get this to work.

dimer.mol (84 Bytes) (1.45 KB)

fix gcmc does not work with fix rigid/small, probably not for fix rigid either. This may eventually get fixed. As an alternative for now, you can still use fix shake. Because this is the second time this has come up recently, I checked a change to the fix_gcmc.cpp code that blocks simultaneous use of fix gcmc and fix rigid or rigid/small. If someone has a legitimate reason to use fix rigid with non-exchange molecules, I can turn off that error check.