I'd like to submit a feature request for Hybrid Monte Carlo (HMC) simulations. At present, one can do mixed MD/MC using a combination of 'fix gcmc' and a time integration fix. However, there's no way to accept/reject the MD moves that take place between the steps when 'fix gcmc' is invoked. I believe that such accept/reject rules are formally necessary to provide detailed balance for proper sampling. (Velocity re-randomization is also necessary, but this can be currently implemented by the user looping between 'velocity create' and 'run' commands.)
I'm not sure of the best implementation for this feature. I think it'd probably be best to add a keyword to 'fix gcmc' to do this, and then LAMMPS would save the state at the end of the Monte Carlo moves and compare it to the energy of the state when the Monte Carlo moves next begin.
I might be able to implement this myself, but I wanted to check first to see if this feature is already being developed and to see what else the developers or other users thought about this.