Alireza,
You will also want to check the auto box for the number of time steps because if you do not then each simulation in your set will have a different simulation time. Think of it this way based on a dt of 6.67128190396304e-18 and 4000 time steps your simulation will be over a time of 26.685 ps. However, based on your settings if you half your grid spacing your dt will become 3.3356e-18 s so if your 4000 is fixed and not on auto this second simulation will be over a time span of 13.343 ps.
Scott