LAMMPS : Potential energy of entire system is coming almost two time as the sum of potential energy of individual atoms in the system. Why ?

03 March 2016 8 4K Report

LAMMPS : Potential energy of entire system is coming almost two time as the sum of potential energy of individual atoms in the system calculated by

compute 3 all pe/atom

compute 4 all reduce sum c_3

commands.

Physically both should be equal in magnitude.

Here I am writing the input commands file.

___________________________________________________

fix 1 all nve

fix 2 all temp/rescale 20 2000.0 2000.0 10.0 1.0

timestep 1.0

thermo 1

run 1000

unfix 2

compute 3 all pe/atom

compute 4 all reduce sum c_3

thermo_style custom step temp pe etotal c_4

dump 20 all custom 10 Trajectory_nve_2000 id xu yu zu vx vy vz c_3

run 1000

____________________________________________________________

OUTPUT of thermo_style

Step Temp PotEng TotEng c_4

1000 1996.2558 -1459674.8 -1445209.2 -748980.04

1001 1997.0884 -1459680.7 -1445209.1 -748987.39

______________________________________________________________

Now just see the 3rd and the last column of the output. The 3rd column is the PE of entire system and the last column is the sum of the PE of individual atoms. These two quantity should be equal but they differs by a multiplicative factor of approximately two. I don't understand why ? Can anybody help me to resolve the issue ?

Omkar

Anna Bozhenko

Do you count the interaction energy between the atoms?

Hegoi Manzano

Could you show the force field you are using? Lennard Jones, coulomb...

Yancong Feng

The value of PotEng is the sum of pair, bond, angle, dihedral.

For the "compute pe command", if no extra keywords are listed, then the potential energy is the sum of pair, bond, angle, dihedral, improper, and kspace (long-range) energy (see manual). So, I think may be the value of improper or kspace make the PotEng inequal to c_4.

Omkar Tripathy

Thank you all for your kind responses. I have already resolve the problem. The issue was with the version of LAMMPS I was using.

Thank u all.

Zi Yu Cong

can you show me the reason? i also don's understand the issue

Sorry for the late answer. I was not in touch. The old version has an issue with the energy calculation. You can use any LAMMPS version of 2016 or 2017.

Digamber Shinde

Sir how to make/get the potential file of NiTi alloy ?

Alberto Fraile

You can find a MEAM potential for Ni-Ti in the NIST repository

https://www.ctcms.nist.gov/potentials/Ti.html

Best

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