Keeping unit cell constant during gfnff geometry

[MasKlein-1]

I was wondering if it is possible to do geometry optimizations under periodic boundary conditions using gfnff while not changing the unit cell parameters. From what I have gathered, gfnff is the only method that supports periodic boundary conditions natively but I have not seen anything that will allow me to fix the unit cell.

Thanks for the help!

[marcelmbn]

@Thomas3R might be able to help as he implemented GFN-FF with periodic boundary conditions.