/* ----------------------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   https://www.lammps.org/, Sandia National Laboratories
   LAMMPS development team: developers@lammps.org

   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.
------------------------------------------------------------------------- */

#include "fix_wall_lj126.h"

#include "atom.h"
#include "error.h"
#include "math_special.h"

using namespace LAMMPS_NS;
using MathSpecial::powint;

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

FixWallLJ126::FixWallLJ126(LAMMPS *lmp, int narg, char **arg) : FixWall(lmp, narg, arg)
{
  dynamic_group_allow = 1;
}

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

void FixWallLJ126::precompute(int m)
{
  coeff1[m] = 48.0 * epsilon[m] * powint(sigma[m], 12);
  coeff2[m] = 24.0 * epsilon[m] * powint(sigma[m], 6);
  coeff3[m] = 4.0 * epsilon[m] * powint(sigma[m], 12);
  coeff4[m] = 4.0 * epsilon[m] * powint(sigma[m], 6);

  double r2inv = 1.0 / (cutoff[m] * cutoff[m]);
  double r6inv = r2inv * r2inv * r2inv;
  offset[m] = r6inv * (coeff3[m] * r6inv - coeff4[m]);
}

/* ----------------------------------------------------------------------
   interaction of all particles in group with a wall
   m = index of wall coeffs
   which = xlo,xhi,ylo,yhi,zlo,zhi
   error if any particle is on or behind wall
------------------------------------------------------------------------- */

void FixWallLJ126::wall_particle(int m, int which, double coord)
{
  double delta, rinv, r2inv, r6inv, fwall;
  double vn;

  double **x = atom->x;
  double **f = atom->f;
  int *mask = atom->mask;
  int nlocal = atom->nlocal;

  int dim = which / 2;
  int side = which % 2;
  if (side == 0) side = -1;

  int onflag = 0;

  for (int i = 0; i < nlocal; i++)
    if (mask[i] & groupbit) {
      if (side < 0)
        delta = x[i][dim] - coord;
      else
        delta = coord - x[i][dim];
      if (delta >= cutoff[m]) continue;
      if (delta <= 0.0) {
        onflag = 1;
        continue;
      }
      rinv = 1.0 / delta;
      r2inv = rinv * rinv;
      r6inv = r2inv * r2inv * r2inv;
      fwall = side * r6inv * (coeff1[m] * r6inv - coeff2[m]) * rinv;
      f[i][dim] -= fwall;
      ewall[0] += r6inv * (coeff3[m] * r6inv - coeff4[m]) - offset[m];
      ewall[m + 1] += fwall;

      if (evflag) {
        if (side < 0)
          vn = -fwall * delta;
        else
          vn = fwall * delta;
        v_tally(dim, i, vn);
      }
    }

  if (onflag) error->one(FLERR, "Particle on or inside fix wall surface");
}