molecules go out of the cell after dynamics in Forcite

Hi every one
I used Amorphous Cell to bulit cell containing 100 molecules and the geometry optimization has been performed after construction.Then the system went through NVT dynamics in Forcite with thermostat Nose.However,lots of molecules went out of the cell after the dynamics finished, so I think the boundary condition does not work,but I do not know what is wrong with my caculation.I come across the same problem when using GULP for dynamics.Picture is  finall frame.

Any help will be appreciated.
Thanks in advance 


Sincerely 
Li