[lammps-users] Question about unit setting

Dear All

Now I am doing some MD simulation on SWNT. I have some questions about unit selection. My input file is list below. My understanding is that there is no difference between lj unit and real unit if I keep consistent between them. But it is not.

When I select real unit, the program works well and the tempreture keeps near 0.1K. But if I use lj unit, the program can not continue to run because the temperature is too high. I know the default timesteps are different for lj and real units. So I set the timestep before I run dynamic but the error still exists. Would you please tell me why I can not get the same results when I use real or lj unit? Thanks.

Best wishes, Lu.

My input script:

processors 1 1 1
units real (lj)
dimension 3
boundary p p p
atom_style molecular
bond_style class2
angle_style class2
dihedral_style class2
improper_style class2
read_data SWNT.data
pair_style lj/class2 12.0
pair_coeff * * 0.068 3.915 12.0
special_bonds lj 0.0 0.0 1.0
neighbor 0.5 bin
group edge type 2
group mobile subtract all edge
timestep 1
fix 1 mobile nvt 0.1 0.1 100
fix 2 edge setforce 0.0 0.0 0.0
dump 1 all xyz 10000 dyna1.xyz
thermo 1
run 50000

Every input parameter in your script and data file will
change if you use real versus lj units. If you do it
all correctly, then you will get identical answers. If you
don't then you'll get different answers.