[lammps-users] Puzzling about pair energy

Dear all

Now I am using a very simple to check the energy output in LAMMPS. The input and data files are listed in the end. My model is about only two carbon atoms. After minimizatoin, the distance between atoms is close to the equilibrium distance, 3.915. But the Final total energy is -0.034, just half of the theoretical result (-0.068).

Energy initial, next-to-last, final =
-0.00446380037908 -0.034 -0.034

I don’t know why there is a difference between MD and Theory. Please give me some advice and thanks so much.

Regards, Lu.

The input file is

processors 1 1 1
units lj
dimension 2
boundary p p p
atom_style atomic
read_data net.data
pair_style lj/class2 10.0
pair_coeff * * 0.068 3.915 10.0
neighbor 0.3 bin
neigh_modify delay 5 every 1
fix 1 all enforce2d
dump 1 all xyz 1 dyna.xyz
minimize 1.0e-12 0.0 100000 100000

the net.data file is

2 atoms
1 atom types

-100.00000 100.00000 xlo xhi
-100.00000 100.00000 ylo yhi
-100.00000 100.00000 zlo zhi

1 12.000
1 1 0.0000000E+00 0.0000000E+00 0.0000000E+00
2 1 6.417000 0.0000000E+00 0.0000000E+00

Thermo output in lj units is normalized by the number
of atoms (by default, see the thermo_modify command),
which in this case will divide by 2.