/* ----------------------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   http://lammps.sandia.gov, Sandia National Laboratories
   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2003) Sandia Corporation.  Under the terms of Contract
   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
   certain rights in this software.  This software is distributed under
   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */

/* ----------------------------------------------------------------------
   Contributing author: Ariel Lozano (arielzn@gmail.com)
   References: Fennell and Gezelter, JCP 124, 234104 (2006)
------------------------------------------------------------------------- */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "pair_born_coul_dsf_cs.h"
#include "atom.h"
#include "comm.h"
#include "force.h"
#include "neighbor.h"
#include "neigh_list.h"
#include "math_const.h"
#include "memory.h"
#include "error.h"
#include "math_special.h"

using namespace LAMMPS_NS;
using namespace MathConst;

#define EPSILON 1.0e-20


/* ---------------------------------------------------------------------- */

PairBornCoulDSFCS::PairBornCoulDSFCS(LAMMPS *lmp) : PairBornCoulDSF(lmp)
{
  writedata = 1;
  single_enable = 0;
}

/* ---------------------------------------------------------------------- */

void PairBornCoulDSFCS::compute(int eflag, int vflag)
{
  int i,j,ii,jj,inum,jnum,itype,jtype;
  double qtmp,xtmp,ytmp,ztmp,delx,dely,delz,evdwl,ecoul,fpair;
  double r,rsq,r2inv,r6inv,forcecoul,forceborn,factor_coul,factor_lj;
  double prefactor,erfcc,erfcd,arg;
  double rexp;
  int *ilist,*jlist,*numneigh,**firstneigh;

  evdwl = ecoul = 0.0;
  if (eflag || vflag) ev_setup(eflag,vflag);
  else evflag = vflag_fdotr = 0;

  double **x = atom->x;
  double **f = atom->f;
  double *q = atom->q;
  int *type = atom->type;
  int nlocal = atom->nlocal;
  double *special_lj = force->special_lj;
  double *special_coul = force->special_coul;
  int newton_pair = force->newton_pair;
  double qqrd2e = force->qqrd2e;

  inum = list->inum;
  ilist = list->ilist;
  numneigh = list->numneigh;
  firstneigh = list->firstneigh;

  // loop over neighbors of my atoms

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

  // self coulombic energy
    if (eflag) {
      double e_self = -(e_shift/2.0 + alpha/MY_PIS) * qtmp*qtmp*qqrd2e;
      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_lj = special_lj[sbmask(j)];
      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;
      jtype = type[j];

      if (rsq < cutsq[itype][jtype]) {
        rsq += EPSILON; // Add Epsilon for case: r = 0; Interaction must be removed by special bond;
        r2inv = 1.0/rsq;

        if (rsq < cut_coulsq) {
          r = sqrt(rsq);
          prefactor = qqrd2e*qtmp*q[j] / r;
	  arg = alpha * r ;
          erfcd = MathSpecial::expmsq(arg);
          erfcc = MathSpecial::my_erfcx(arg) * erfcd;
          forcecoul = prefactor * (erfcc/r + 2.0*alpha/MY_PIS * erfcd +
                                   r*f_shift) * r;
          if (factor_coul < 1.0) forcecoul -= (1.0-factor_coul)*prefactor;
        } else forcecoul = 0.0;

        if (rsq < cut_ljsq[itype][jtype]) {
          r6inv = r2inv*r2inv*r2inv;
          r = sqrt(rsq);
          rexp = exp((sigma[itype][jtype]-r)*rhoinv[itype][jtype]);
          forceborn = born1[itype][jtype]*r*rexp - born2[itype][jtype]*r6inv
            + born3[itype][jtype]*r2inv*r6inv;
        } else forceborn = 0.0;

        fpair = (forcecoul + factor_lj*forceborn) * r2inv;

        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) {
          if (rsq < cut_coulsq) {
            ecoul = prefactor * (erfcc - r*e_shift - rsq*f_shift);
            if (factor_coul < 1.0) ecoul -= (1.0-factor_coul)*prefactor;
          } else ecoul = 0.0;
          if (rsq < cut_ljsq[itype][jtype]) {
            evdwl = a[itype][jtype]*rexp - c[itype][jtype]*r6inv +
              d[itype][jtype]*r6inv*r2inv - offset[itype][jtype];
            evdwl *= factor_lj;
          } else evdwl = 0.0;
        }

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

  if (vflag_fdotr) virial_fdotr_compute();
}

