# Tersoff potential

Hello,

I try to use the tersoff potential with lammps. In order to test and understand how lammps works, I want to calculate the potential energy of the 3-atom system with the coordinates 0 0 0, 1 0 0, -1 0 0.

I set the repulsive energy to zero (A=0), the angular term g=1, the attractive prefactor B=1, lambda_2=0, beta=1, n=1, m=1, and finally lambda_3=1. I make sure that the cutoff function = 1 for all atom pairs, and the size of the simulation box is large enough so that there are no interactions via the periodic boundaries.

Everything is in “metal” units and the variables I mentioned are defined here: pair_style tersoff command — LAMMPS documentation

In my understanding, the total energy for the central atom should then be -1/sqrt(2) [zeta=1 because rij=rik=1], however lammps says it is -0.781063.

Can someone please explain to me why? It seems so simple, but I don’t see the problem with my calculation, nor with the lammps script. I’d be grateful for any advice.

Here is the corresponding lammps script:

``````units                   metal

region                  mybox block -10 10 -10 10 -10 10
create_box              1 mybox

create_atoms            1 single 0.0 0.0 0.0
create_atoms            1 single 1.0 0.0 0.0
create_atoms            1 single -1.0 0.0 0.0

mass                    1 1

pair_style              tersoff
pair_coeff              * * test.tersoff C

compute                 poten all pe/atom
dump                    1 all custom 1 dump.dat type x y z c_poten
run                     0
``````

And the test.tersoff file:

``````# el1, el2, el3, m, gamma, lambda3, c, d, h, n, beta, lambda2, B, R, D, lambda1, A
C C C 1 1.0 1.0 0.0 1.0 0.0 1.0 1.0 0.0 1.0 2.2 0.1 0.0 0.0
``````

Ok, never mind, sorry. I understand now. In case someone has the same problem: It is the per atom energy that is not calculated in the way I thought it would be, see https://docs.lammps.org/compute_pe_atom.html

For an energy contribution produced by a small set of atoms (e.g. 4 atoms in a dihedral or 3 atoms in a Tersoff 3-body interaction), that energy is assigned in equal portions to each atom in the set. E.g. 1/4 of the dihedral energy to each of the 4 atoms.

The total energy is just what I would have expected it to be.