simulation box increasing when using hybrid granular molecular

Hi ,

I tried to use hybrid granular molecular to simulate multisphere systems, the atoms are successfully read, and I used fix rigid infile to define the center of mass, and moments of inertia. Initially, the non-spherical bodies are uniformly distributed in the simulation box, with a volume fraction of about 0.1, and the first few thousand time steps are running fine (time step = 1e-6), however, the simulation box will increasing after several thousand of time steps, and I can see the total amount of atoms are decreasing.

I monitored the Pzz, and it's increasing rapidly while I also see the simulation box (vol) increases dramatically. Also, I checked the animations, and this problem occurs when the volume fractions of these non-spheres bodies gets larger, say 0.3 ? BTW, I've tried to reduce the time step to 2e-7, but the problem still occurs almost at the same time ( ie. number of time steps x time_step)

The following is my input script, any hint to fix this problem?

#######################################################
....
atom_style hybrid granular molecular
atom_modify map array
comunicate single vel yes

boundary p p m

read_data data.txt
....
neighbor 0.001 bin
neighbor delay 0
neighbor exclude molecular all
.....
pair_style gran model hertz tangential history
pair_coeff * *
.......

fix 1 all rigid molecular infile rigiddata.txt

.......
#################################################
Here is the section of monitoring the pzz and vol

Step Atom Pzz Volume
1 43248 307.10302 1.5e-05
...

6000 42033 28702996 0.00022867619

Any help will be highly appreciated, Thanks!

Best