QM/MM with isolated QM box

[shihao]

Hi there,

I reproduced the error with a smaller system (2 QM water and 21 MM
water).  When I tried to further reduce the system and use only 1 QM
water, the simulation worked fine and the QM water travelled freely.

I've uploaded all the needed files (in qmmm_water_shihao_tar_gz.zip).
If you run the simulation, you will see that the QM water molecules
(resid 1 and 2) more or less stay in the YZ plane (which means that
they could not rotate freely in all directions) after about 200 steps.
I've also included some result files (dcd, cube, etc.) for your
convenience.

Could you give me some clues on whether this is because of some errors
in my input file or some bugs in the code?

Many thanks!

Regards,
Shihao