[lammps-users] Question about Source file(pair_tersoff.cpp & pair.cpp)

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1

Dear all

According to /src/pair_tersoff.cpp , LAMMPS derives every atoms’ stress via differentiating potential energy, which seems natural for that subject. However, it’s v_tally3 function in pair.cpp ,which is internally called during the stress calculation, that puzzles me.

Specifically, at first, I cannot understand why i,j,k-indexed atoms (the 3 atoms involved in calculation) shares equal stress. I think the stress of atom i (at the center of these 3) should be bigger than two others, since the force of it is sum of the others.

Secondly, I cannot figure out why THIRD term appears here. I think this term is to avoid double-summation(or excessive summation), likewise on ev_tally function, but I think it should be just 1/2, because only j and k can be switched during permutation.

These might be silly questions, but is anyone have suggestions in dealing these problems?
(I looked up papers regarding this matter such as from ‘Izumi et al’ on thin solid film journals on 2004, but it didn’t help.)

Thanks in advance

v_tally3 funtions are below::
v[0] = THIRD * (drik[0]*fi[0] + drjk[0]*fj[0]);

  v[1] = THIRD * (drik[1]*fi[1] + drjk[1]*fj[1]);
  v[2] = THIRD * (drik[2]*fi[2] + drjk[2]*fj[2]);
  v[3] = THIRD * (drik[0]*fi[1] + drjk[0]*fj[1]);
  v[4] = THIRD * (drik[0]*fi[2] + drjk[0]*fj[2]);
  v[5] = THIRD * (drik[1]*fi[2] + drjk[1]*fj[2]);

  vatom[i][0] += v[0]; vatom[i][1] += v[1]; vatom[i][2] += v[2];
  vatom[i][3] += v[3]; vatom[i][4] += v[4]; vatom[i][5] += v[5];
  vatom[j][0] += v[0]; vatom[j][1] += v[1]; vatom[j][2] += v[2];
  vatom[j][3] += v[3]; vatom[j][4] += v[4]; vatom[j][5] += v[5];
  vatom[k][0] += v[0]; vatom[k][1] += v[1]; vatom[k][2] += v[2];
  vatom[k][3] += v[3]; vatom[k][4] += v[4]; vatom[k][5] += v[5];

}

2

Dear all
According to /src/pair_tersoff.cpp , LAMMPS derives every atoms' stress via
differentiating potential energy, which seems natural for that subject.
However, it's v_tally3 function in pair.cpp ,which is internally called
during the stress calculation, that puzzles me.
Specifically, at first, I cannot understand why i,j,k-indexed atoms (the 3
atoms involved in calculation) shares equal stress. I think the stress of
atom i (at the center of these 3) should be bigger than two others, since
the force of it is sum of the others.

no!

v_tally3 is called to add stress contribution for 3-body terms.

the pairwise additive terms are already handles by ev_tally() earlier on.

Secondly, I cannot figure out why THIRD term appears here. I think this term
is to avoid double-summation(or excessive summation), likewise on ev_tally
function, but I think it should be just 1/2, because only j and k can be
switched during permutation.

since all three atoms contribute to the stress from a 3-body term,
they all get a third. check out the Angle::ev_tally() method in angle.cpp
for comparison.

These might be silly questions, but is anyone have suggestions in dealing
these problems?

many-body potentials are different. :wink:

cheers,
   axel.