diff --git a/extra/fix_addbornforce.cpp b/extra/fix_addbornforce.cpp
new file mode 100644
index 0000000..e586a7d
--- /dev/null
+++ b/extra/fix_addbornforce.cpp
@@ -0,0 +1,405 @@
+/* ----------------------------------------------------------------------
+   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_addbornforce.h"
+
+#include "atom.h"
+#include "atom_masks.h"
+#include "comm.h"
+#include "domain.h"
+#include "error.h"
+#include "input.h"
+#include "memory.h"
+#include "force.h"
+#include "modify.h"
+#include "region.h"
+#include "update.h"
+#include "variable.h"
+#include "pair_allegro.h"
+#include <torch/torch.h>
+#include <iostream>
+
+#include <cstring>
+
+using namespace LAMMPS_NS;
+using namespace FixConst;
+
+enum { NONE, CONSTANT, EQUAL, ATOM };
+
+/* ---------------------------------------------------------------------- */
+/*
+ * fix ID group-ID addbornforce ex ey ez keyword value
+ */
+
+FixAddBornForce::FixAddBornForce(LAMMPS *lmp, int narg, char **arg)
+    : Fix(lmp, narg, arg), xstr(nullptr), ystr(nullptr), zstr(nullptr),
+      idregion(nullptr), region(nullptr), efieldatom(nullptr) {
+  if (narg < 6)
+    utils::missing_cmd_args(FLERR, "fix addbornforce", error);
+
+  dynamic_group_allow = 1;
+  scalar_flag = 1;
+  vector_flag = 1;
+  size_vector = 3;
+  global_freq = 1;
+  extscalar = 1;
+  extvector = 1;
+  energy_global_flag = 1;
+  virial_global_flag = virial_peratom_flag = 0;
+
+  if (utils::strmatch(arg[3], "^v_")) {
+    xstr = utils::strdup(arg[3] + 2);
+  } else {
+    xvalue = utils::numeric(FLERR, arg[3], false, lmp);
+    xstyle = CONSTANT;
+  }
+  if (utils::strmatch(arg[4], "^v_")) {
+    ystr = utils::strdup(arg[4] + 2);
+  } else {
+    yvalue = utils::numeric(FLERR, arg[4], false, lmp);
+    ystyle = CONSTANT;
+  }
+  if (utils::strmatch(arg[5], "^v_")) {
+    zstr = utils::strdup(arg[5] + 2);
+  } else {
+    zvalue = utils::numeric(FLERR, arg[5], false, lmp);
+    zstyle = CONSTANT;
+  }
+
+  // always called, makes no sense to have nevery > 1
+  nevery = 1;
+
+  int iarg = 6;
+  while (iarg < narg) {
+    if (strcmp(arg[iarg], "region") == 0) {
+      if (iarg + 2 > narg)
+        utils::missing_cmd_args(FLERR, "fix addbornforce region", error);
+      region = domain->get_region_by_id(arg[iarg + 1]);
+      if (!region)
+        error->all(FLERR, "Region {} for fix addbornforce does not exist",
+                   arg[iarg + 1]);
+      idregion = utils::strdup(arg[iarg + 1]);
+      iarg += 2;
+    } else
+      error->all(FLERR, "Unknown fix addbornforce keyword: {}", arg[iarg]);
+  }
+
+  reduced_flag = 0;
+
+  maxatom = 1;
+  memory->create(efieldatom, maxatom, 3, "addbornforce:efieldatom");
+
+  // KOKKOS package
+
+  datamask_read = X_MASK | F_MASK | MASK_MASK | IMAGE_MASK;
+  datamask_modify = F_MASK;
+
+  ((PairAllegro<lowhigh> *)force->pair)->add_custom_output("born_charge");
+  ((PairAllegro<lowhigh> *)force->pair)->add_custom_output("polarization");
+  ((PairAllegro<lowhigh> *)force->pair)->add_custom_output("polarizability");
+}
+
+/* ---------------------------------------------------------------------- */
+
+FixAddBornForce::~FixAddBornForce() {
+  delete[] xstr;
+  delete[] ystr;
+  delete[] zstr;
+  delete[] idregion;
+  memory->destroy(efieldatom);
+}
+
+/* ---------------------------------------------------------------------- */
+
+int FixAddBornForce::setmask() {
+  int mask = 0;
+  mask |= POST_FORCE;
+  mask |= MIN_POST_FORCE;
+  return mask;
+}
+
+/* ---------------------------------------------------------------------- */
+
+void FixAddBornForce::init() {
+  // check variables
+
+  if (xstr) {
+    xvar = input->variable->find(xstr);
+    if (xvar < 0)
+      error->all(FLERR, "Variable {} for fix addbornforce does not exist",
+                 xstr);
+    if (input->variable->equalstyle(xvar))
+      xstyle = EQUAL;
+    else if (input->variable->atomstyle(xvar))
+      xstyle = ATOM;
+    else
+      error->all(FLERR, "Variable {} for fix addbornforce is invalid style",
+                 xstr);
+  }
+  if (ystr) {
+    yvar = input->variable->find(ystr);
+    if (yvar < 0)
+      error->all(FLERR, "Variable {} for fix addbornforce does not exist",
+                 ystr);
+    if (input->variable->equalstyle(yvar))
+      ystyle = EQUAL;
+    else if (input->variable->atomstyle(yvar))
+      ystyle = ATOM;
+    else
+      error->all(FLERR, "Variable {} for fix addbornforce is invalid style",
+                 ystr);
+  }
+  if (zstr) {
+    zvar = input->variable->find(zstr);
+    if (zvar < 0)
+      error->all(FLERR, "Variable {} for fix addbornforce does not exist",
+                 zstr);
+    if (input->variable->equalstyle(zvar))
+      zstyle = EQUAL;
+    else if (input->variable->atomstyle(zvar))
+      zstyle = ATOM;
+    else
+      error->all(FLERR, "Variable {} for fix addbornforce is invalid style",
+                 zstr);
+  }
+
+  // set index and check validity of region
+
+  if (idregion) {
+    region = domain->get_region_by_id(idregion);
+    if (!region)
+      error->all(FLERR, "Region {} for fix addbornforce does not exist",
+                 idregion);
+  }
+
+  if (xstyle == ATOM || ystyle == ATOM || zstyle == ATOM)
+    varflag = ATOM;
+  else if (xstyle == EQUAL || ystyle == EQUAL || zstyle == EQUAL)
+    varflag = EQUAL;
+  else
+    varflag = CONSTANT;
+
+  if (varflag != CONSTANT)
+    error->warning(FLERR, "non-constant efield is experimental, strongly distrust scalar/vector output");
+}
+
+/* ---------------------------------------------------------------------- */
+
+void FixAddBornForce::setup(int vflag) {
+  if (utils::strmatch(update->integrate_style, "^verlet"))
+    post_force(vflag);
+  else {
+    error->all(FLERR, "unsupported integration");
+  }
+}
+
+/* ---------------------------------------------------------------------- */
+
+void FixAddBornForce::min_setup(int vflag) { post_force(vflag); }
+
+/* ---------------------------------------------------------------------- */
+
+void FixAddBornForce::post_force(int vflag) {
+  double **x = atom->x;
+  double **f = atom->f;
+  int *mask = atom->mask;
+  imageint *image = atom->image;
+  double v[6];
+  int nlocal = atom->nlocal;
+
+  if (update->ntimestep % nevery)
+    return;
+
+  // virial setup
+
+  v_init(vflag);
+
+  // update region if necessary
+
+  if (region)
+    region->prematch();
+
+  // reallocate efieldatom array if necessary
+
+  if (varflag == ATOM && atom->nmax > maxatom) {
+    maxatom = atom->nmax;
+    memory->destroy(efieldatom);
+    memory->create(efieldatom, maxatom, 3, "addbornforce:efieldatom");
+  }
+
+  reduced_flag = 0;
+  extraenergy = extrapolarization[0] = extrapolarization[1] = extrapolarization[2] = 0.0;
+
+  torch::Tensor born_tensor =
+    ((PairAllegro<lowhigh> *) force->pair)->custom_output.at("born_charge").cpu();
+  this->born_tensor = born_tensor;
+
+  auto born = born_tensor.accessor<double,3>();
+
+  // reverse comm and add up Born charges, Newton-style
+  comm->reverse_comm(this, 9);
+
+  // constant force
+  // potential energy = - x dot f in unwrapped coords
+
+  if (varflag == CONSTANT) {
+    double unwrap[3];
+    for (int i = 0; i < nlocal; i++)
+      if (mask[i] & groupbit) {
+        if (region && !region->match(x[i][0], x[i][1], x[i][2]))
+          continue;
+        domain->unmap(x[i], image[i], unwrap);
+        // row-vector efield * Born charge matrix
+        // since Zij = dFj/dei = dPi/drj
+        for (int j = 0; j < 3; j++) {
+          f[i][j] += xvalue * born[i][0][j] + yvalue*born[i][1][j] + zvalue*born[i][2][j];
+        }
+      }
+
+    // variable force, wrap with clear/add
+    // potential energy = evar if defined, else 0.0
+    // wrap with clear/add
+
+  } else {
+    // TODO: Figure out extra energy/polarization in this case
+    double unwrap[3];
+
+    modify->clearstep_compute();
+
+    if (xstyle == EQUAL)
+      xvalue = input->variable->compute_equal(xvar);
+    else if (xstyle == ATOM)
+      input->variable->compute_atom(xvar, igroup, &efieldatom[0][0], 3, 0);
+    if (ystyle == EQUAL)
+      yvalue = input->variable->compute_equal(yvar);
+    else if (ystyle == ATOM)
+      input->variable->compute_atom(yvar, igroup, &efieldatom[0][1], 3, 0);
+    if (zstyle == EQUAL)
+      zvalue = input->variable->compute_equal(zvar);
+    else if (zstyle == ATOM)
+      input->variable->compute_atom(zvar, igroup, &efieldatom[0][2], 3, 0);
+
+    modify->addstep_compute(update->ntimestep + 1);
+
+    for (int i = 0; i < nlocal; i++) {
+      if (mask[i] & groupbit) {
+        if (region && !region->match(x[i][0], x[i][1], x[i][2]))
+          continue;
+        domain->unmap(x[i], image[i], unwrap);
+        if (xstyle == ATOM)
+          xvalue = efieldatom[i][0];
+        if (ystyle == ATOM)
+          yvalue = efieldatom[i][1];
+        if (zstyle == ATOM)
+          zvalue = efieldatom[i][2];
+
+        // row-vector efield * Born charge matrix
+        // since Zij = dFj/dei = dPi/drj
+        for (int j = 0; j < 3; j++) {
+          f[i][j] += xvalue * born[i][0][j] + yvalue*born[i][1][j] + zvalue*born[i][2][j];
+        }
+      }
+    }
+  }
+
+
+  torch::Tensor polarization_tensor =
+    ((PairAllegro<lowhigh> *) force->pair)->custom_output.at("polarization").cpu();
+  torch::Tensor polarizability_tensor =
+    ((PairAllegro<lowhigh> *) force->pair)->custom_output.at("polarizability").cpu();
+
+  // std::cout << "polarization: " << polarization_tensor << std::endl;
+  // std::cout << "polarizability: " << polarizability_tensor << std::endl;
+
+  auto polarization = polarization_tensor.accessor<double, 2>();
+  auto polarizability = polarizability_tensor.accessor<double, 3>();
+
+  for (int i = 0; i <3; i++) {
+    extrapolarization[i] = xvalue*polarizability[0][0][i] + yvalue*polarizability[0][1][i] + zvalue*polarizability[0][2][i];
+  }
+
+  extraenergy = -(xvalue * (polarization[0][0]+extrapolarization[0])
+                + yvalue * (polarization[0][1]+extrapolarization[1])
+                + zvalue * (polarization[0][2]+extrapolarization[2]));
+
+}
+
+/* ---------------------------------------------------------------------- */
+
+void FixAddBornForce::min_post_force(int vflag) { post_force(vflag); }
+
+int FixAddBornForce::pack_reverse_comm(int n, int first, double *buf){
+  auto born = born_tensor.accessor<double,3>();
+  int m = 0;
+  int last = first + n;
+
+  for (int i = first; i < last; i++) {
+    for (int j = 0; j < 3; j++) {
+      for (int k = 0; k < 3; k++) {
+        buf[m++] = born[i][j][k];
+      }
+    }
+  }
+  return m;
+}
+
+void FixAddBornForce::unpack_reverse_comm(int n, int *list, double *buf){
+  auto born = born_tensor.accessor<double,3>();
+  int m = 0;
+
+  for (int i = 0; i < n; i++) {
+    int ii = list[i];
+    for (int j = 0; j < 3; j++) {
+      for (int k = 0; k < 3; k++) {
+        born[ii][j][k] += buf[m++];
+      }
+    }
+  }
+}
+
+/* ----------------------------------------------------------------------
+   potential energy of added force
+------------------------------------------------------------------------- */
+
+double FixAddBornForce::compute_scalar() {
+  // only sum across procs one time
+
+  double tmp = 0.0;
+  MPI_Allreduce(&extraenergy, &tmp, 1, MPI_DOUBLE, MPI_SUM, world);
+  return tmp;
+}
+
+/* ----------------------------------------------------------------------
+   return components of total force on fix group before force was changed
+------------------------------------------------------------------------- */
+
+double FixAddBornForce::compute_vector(int n) {
+  // only sum across procs one time
+
+  if (reduced_flag == 0) {
+    MPI_Allreduce(MPI_IN_PLACE, extrapolarization, 4, MPI_DOUBLE, MPI_SUM, world);
+    reduced_flag = 1;
+  }
+  return extrapolarization[n];
+}
+
+/* ----------------------------------------------------------------------
+   memory usage of local atom-based array
+------------------------------------------------------------------------- */
+
+double FixAddBornForce::memory_usage() {
+  double bytes = 0.0;
+  if (varflag == ATOM)
+    bytes = maxatom * 3 * sizeof(double);
+  return bytes;
+}
diff --git a/extra/fix_addbornforce.h b/extra/fix_addbornforce.h
new file mode 100644
index 0000000..9d31a50
--- /dev/null
+++ b/extra/fix_addbornforce.h
@@ -0,0 +1,65 @@
+/* -*- c++ -*- ----------------------------------------------------------
+   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.
+------------------------------------------------------------------------- */
+
+#ifdef FIX_CLASS
+// clang-format off
+FixStyle(addbornforce,FixAddBornForce);
+// clang-format on
+#else
+
+#ifndef LMP_FIX_ADDBORNFORCE_H
+#define LMP_FIX_ADDBORNFORCE_H
+
+#include "fix.h"
+#include <torch/torch.h>
+
+namespace LAMMPS_NS {
+
+class FixAddBornForce : public Fix {
+public:
+  FixAddBornForce(class LAMMPS *, int, char **);
+  ~FixAddBornForce() override;
+  int setmask() override;
+  void init() override;
+  void setup(int) override;
+  void min_setup(int) override;
+  void post_force(int) override;
+  void min_post_force(int) override;
+  double compute_vector(int) override;
+  double memory_usage() override;
+
+  // this is used as the energy contribution
+  double compute_scalar() override;
+
+  int pack_reverse_comm(int, int, double *) override;
+  void unpack_reverse_comm(int, int *, double *) override;
+
+private:
+  double xvalue, yvalue, zvalue;
+  int varflag;
+  char *xstr, *ystr, *zstr;
+  char *idregion;
+  class Region *region;
+  int xvar, yvar, zvar, xstyle, ystyle, zstyle;
+  int reduced_flag;
+  double extrapolarization[3], extraenergy;
+
+  int maxatom;
+  double **efieldatom;
+  torch::Tensor born_tensor;
+};
+
+} // namespace LAMMPS_NS
+
+#endif
+#endif