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[lammps-users] Pair_style coul/wolf


From: Kousika A <kousika12@...24...>
Date: Fri, 11 Aug 2017 15:04:38 +0530

Dear all,
             I need some clarification in the c++ file of pair_style coul/wolf as I would like to modify this a bit and use a modified potential. 

A part from the program is given below,

for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    qtmp = q[i];
    xtmp = x[i][0];
    ytmp = x[i][1];
    ztmp = x[i][2];
    jlist = firstneigh[i];
    jnum = numneigh[i];

    qisq = qtmp*qtmp;
    e_self = -(e_shift/2.0 + alf/MY_PIS) * qisq*qqrd2e;
    if (evflag) ev_tally(i,i,nlocal,0,0.0,e_self,0.0,0.0,0.0,0.0);

    for (jj = 0; jj < jnum; jj++) {
      j = jlist[jj];
      factor_coul = special_coul[sbmask(j)];
      j &= NEIGHMASK;

      delx = xtmp - x[j][0];
      dely = ytmp - x[j][1];
      delz = ztmp - x[j][2];
      rsq = delx*delx + dely*dely + delz*delz;

      if (rsq < cut_coulsq) {
        r = sqrt(rsq);
        prefactor = qqrd2e*qtmp*q[j]/r;
        erfcc = erfc(alf*r);
        erfcd = exp(-alf*alf*r*r);
        v_sh = (erfcc - e_shift*r) * prefactor;
        dvdrr = (erfcc/rsq + 2.0*alf/MY_PIS * erfcd/r) + f_shift;
        forcecoul = dvdrr*rsq*prefactor;
        if (factor_coul < 1.0) forcecoul -= (1.0-factor_coul)*prefactor;
        fpair = forcecoul / rsq;

        f[i][0] += delx*fpair;
        f[i][1] += dely*fpair;
        f[i][2] += delz*fpair;
        if (newton_pair || j < nlocal) {
          f[j][0] -= delx*fpair;
          f[j][1] -= dely*fpair;
          f[j][2] -= delz*fpair;
        }

        if (eflag) {
          ecoul = v_sh;
          if (factor_coul < 1.0) ecoul -= (1.0-factor_coul)*prefactor;
        } else ecoul = 0.0;

        if (evflag) ev_tally(i,j,nlocal,newton_pair,
                             0.0,ecoul,fpair,delx,dely,delz);
      }
    }
  } 


My questions are 

1)What is v_sh calculating? Is that the force term? 

2) And in the next step dvdrr, erfcc is divided by rsq but the next step is divided by r

3)What is the importance of factor_coul in this program?


Kindly clarify this. Thanks a lot in advance.

--
Best regards,
A.KOUSIKA
Research Scholar
Department of Metallurgical and Materials Engineering
IITMadras.