[lammps-users] force on atoms with LJ potential

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1

Dear All,

I am working with lj/cut potential.
I used the following commands to output pair list , coordinates of atom , force on atom and distance between pairs of atoms. When I try to calculate fx (force in x-direction) on each atom using LJ 12-6 formula, I was unable to get the ‘fx’ which i got from dump command.
Am i doing anything wrong here?

compute 1 upper property/local patom1 patom2
compute 2 upper pair/local dist force

dump 2 upper custom 10 cfs id x y z fx
dump 5 upper local 10 indices c_1[1] c_1[2]
dump 6 upper local 10 dist_force c_2[1] c_2[2]

Thanks&Regards
Sivashankar
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2

Fx will be the sum of all forces on atom I in your dump atom file.
Shoudl be the sum of all terms involving atom I in your dump local file.

Steve

3

Hi steeve
thanks for the reply.
I have one more question on the LJ force.

I used these commands for LJ Potential.

pair_style lj/cut 2.5
pair_coeff * * 1.0 1.0 2.5

compute 2 upper pair/local dist force eng
dump 6 upper local 10 dist_force c_2[1] c_2[2] c_2[3]

I calculated the forces from the pair distances that i got from the dump command using the expression -4*[-12r^-13 + 6r^-7]. The calculated forces and the forces that i got from the dump command are not matching. But the energy is exactly matching.

pair distance Calculated forces Forces from dump command
1.93527 -0.2271 -0.0117334
2.25307 -0.0802 -0.03557

Can anyone help me

Thanks in advance
Sivashankar

4

Compute pair/local was missing a factor of “r” in the force calculation.
I’ll post a patch this AM. Then see if you get the answers you expect.

Thanks,
Steve