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