// ******************************************************************************
   //
   // Title:       Force Field X.
   // Description: Force Field X - Software for Molecular Biophysics.
   // Copyright:   Copyright (c) Michael J. Schnieders 2001-2025.
   //
   // This file is part of Force Field X.
   //
   // Force Field X is free software; you can redistribute it and/or modify it
  // under the terms of the GNU General Public License version 3 as published by
  // the Free Software Foundation.
  //
  // Force Field X is distributed in the hope that it will be useful, but WITHOUT
  // ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  // FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
  // details.
  //
  // You should have received a copy of the GNU General Public License along with
  // Force Field X; if not, write to the Free Software Foundation, Inc., 59 Temple
  // Place, Suite 330, Boston, MA 02111-1307 USA
  //
  // Linking this library statically or dynamically with other modules is making a
  // combined work based on this library. Thus, the terms and conditions of the
  // GNU General Public License cover the whole combination.
  //
  // As a special exception, the copyright holders of this library give you
  // permission to link this library with independent modules to produce an
  // executable, regardless of the license terms of these independent modules, and
  // to copy and distribute the resulting executable under terms of your choice,
  // provided that you also meet, for each linked independent module, the terms
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  // module which is not derived from or based on this library. If you modify this
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  // ******************************************************************************
  package ffx.numerics.multipole;
  
  import jdk.incubator.vector.DoubleVector;
  
  /**
   * The GeneralizedKirkwoodTensor class contains utilities for generated Generalized Kirkwood
   * interaction tensors.
   *
   * @author Michael J. Schnieders
   * @since 1.0
   */
  public class GKTensorGlobal extends CoulombTensorGlobal {
  
    /**
     * The GK tensor can be constructed for monopoles (GB), dipoles or quadrupoles.
     */
    protected final GKMultipoleOrder multipoleOrder;
  
    /**
     * The Kirkwood dielectric function for the given multipole order.
     */
    private final double c;
  
    private final GKSource gkSource;
  
    /**
     * Construct a new GKTensorGlobal object.
     *
     * @param multipoleOrder The multipole order.
     * @param order          The number of derivatives to complete.
     * @param gkSource       Generate the source terms for the GK recurrence.
     * @param Eh             Homogeneous dielectric constant.
     * @param Es             Solvent dielectric constant.
     */
    public GKTensorGlobal(GKMultipoleOrder multipoleOrder, int order, GKSource gkSource,
                          double Eh, double Es) {
      super(order);
      this.multipoleOrder = multipoleOrder;
      this.gkSource = gkSource;
  
      // Load the dielectric function
      c = GKSource.cn(multipoleOrder.getOrder(), Eh, Es);
    }
  
    /**
     * GK Permanent multipole energy.
     *
     * @param mI PolarizableMultipole at site I.
     * @param mK PolarizableMultipole at site K.
     * @return a double.
     */
    @Override
    public double multipoleEnergy(PolarizableMultipole mI, PolarizableMultipole mK) {
      return switch (multipoleOrder) {
        default -> {
          chargeIPotentialAtK(mI, 2);
          double eK = multipoleEnergy(mK);
          chargeKPotentialAtI(mK, 2);
          double eI = multipoleEnergy(mI);
          yield c * 0.5 * (eK + eI);
        }
        case DIPOLE -> {
         dipoleIPotentialAtK(mI.dx, mI.dy, mI.dz, 2);
         double eK = multipoleEnergy(mK);
         dipoleKPotentialAtI(mK.dx, mK.dy, mK.dz, 2);
         double eI = multipoleEnergy(mI);
         yield c * 0.5 * (eK + eI);
       }
       case QUADRUPOLE -> {
         quadrupoleIPotentialAtK(mI, 2);
         double eK = multipoleEnergy(mK);
         quadrupoleKPotentialAtI(mK, 2);
         double eI = multipoleEnergy(mI);
         yield c * 0.5 * (eK + eI);
       }
     };
   }
 
   /**
    * GK Permanent multipole energy and gradient.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @param Gi Coordinate gradient at site I.
    * @param Gk Coordinate gradient at site K.
    * @param Ti Torque at site I.
    * @param Tk Torque at site K.
    * @return the permanent multipole GK energy.
    */
   @Override
   public double multipoleEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                            double[] Gi, double[] Gk, double[] Ti, double[] Tk) {
     return switch (multipoleOrder) {
       default -> monopoleEnergyAndGradient(mI, mK, Gi, Gk, Ti, Tk);
       case DIPOLE -> dipoleEnergyAndGradient(mI, mK, Gi, Gk, Ti, Tk);
       case QUADRUPOLE -> quadrupoleEnergyAndGradient(mI, mK, Gi, Gk, Ti, Tk);
     };
   }
 
   /**
    * Permanent multipole energy and gradient using the GK monopole tensor.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @param Gi Coordinate gradient at site I.
    * @param Gk Coordinate gradient at site K.
    * @param Ti Torque at site I.
    * @param Tk Torque at site K.
    * @return the permanent multipole GK energy.
    */
   protected double monopoleEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                              double[] Gi, double[] Gk, double[] Ti, double[] Tk) {
 
     // Compute the potential due to a multipole component at site I.
     chargeIPotentialAtK(mI, 3);
     double eK = multipoleEnergy(mK);
     multipoleGradient(mK, Gk);
     multipoleTorque(mK, Tk);
 
     // Compute the potential due to a multipole component at site K.
     chargeKPotentialAtI(mK, 3);
     double eI = multipoleEnergy(mI);
     multipoleGradient(mI, Gi);
     multipoleTorque(mI, Ti);
 
     double scale = c * 0.5;
     Gi[0] = scale * (Gi[0] - Gk[0]);
     Gi[1] = scale * (Gi[1] - Gk[1]);
     Gi[2] = scale * (Gi[2] - Gk[2]);
     Gk[0] = -Gi[0];
     Gk[1] = -Gi[1];
     Gk[2] = -Gi[2];
 
     Ti[0] = scale * Ti[0];
     Ti[1] = scale * Ti[1];
     Ti[2] = scale * Ti[2];
     Tk[0] = scale * Tk[0];
     Tk[1] = scale * Tk[1];
     Tk[2] = scale * Tk[2];
 
     return scale * (eK + eI);
   }
 
   /**
    * Permanent multipole energy and gradient using the GK dipole tensor.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @param Gi Coordinate gradient at site I.
    * @param Gk Coordinate gradient at site K.
    * @param Ti Torque at site I.
    * @param Tk Torque at site K.
    * @return the permanent multipole GK energy.
    */
   protected double dipoleEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                            double[] Gi, double[] Gk, double[] Ti, double[] Tk) {
 
     // Compute the potential due to a multipole component at site I.
     dipoleIPotentialAtK(mI.dx, mI.dy, mI.dz, 3);
     double eK = multipoleEnergy(mK);
     multipoleGradient(mK, Gk);
     multipoleTorque(mK, Tk);
 
     // Need the torque on site I pole due to site K multipole.
     // Only torque on the site I dipole.
     multipoleKPotentialAtI(mK, 1);
     dipoleTorque(mI, Ti);
 
     // Compute the potential due to a multipole component at site K.
     dipoleKPotentialAtI(mK.dx, mK.dy, mK.dz, 3);
     double eI = multipoleEnergy(mI);
     multipoleGradient(mI, Gi);
     multipoleTorque(mI, Ti);
 
     // Need the torque on site K pole due to multipole on site I.
     // Only torque on the site K dipole.
     multipoleIPotentialAtK(mI, 1);
     dipoleTorque(mK, Tk);
 
     double scale = c * 0.5;
     Gi[0] = scale * (Gi[0] - Gk[0]);
     Gi[1] = scale * (Gi[1] - Gk[1]);
     Gi[2] = scale * (Gi[2] - Gk[2]);
     Gk[0] = -Gi[0];
     Gk[1] = -Gi[1];
     Gk[2] = -Gi[2];
 
     Ti[0] = scale * Ti[0];
     Ti[1] = scale * Ti[1];
     Ti[2] = scale * Ti[2];
     Tk[0] = scale * Tk[0];
     Tk[1] = scale * Tk[1];
     Tk[2] = scale * Tk[2];
 
     return scale * (eK + eI);
   }
 
   /**
    * Permanent multipole energy and gradient using the GK quadrupole tensor.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @param Gi Coordinate gradient at site I.
    * @param Gk Coordinate gradient at site K.
    * @param Ti Torque at site I.
    * @param Tk Torque at site K.
    * @return the permanent multipole GK energy.
    */
   protected double quadrupoleEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                                double[] Gi, double[] Gk, double[] Ti, double[] Tk) {
 
     // Compute the potential due to a multipole component at site I.
     quadrupoleIPotentialAtK(mI, 3);
     double eK = multipoleEnergy(mK);
     multipoleGradient(mK, Gk);
     multipoleTorque(mK, Tk);
 
     // Need the torque on site I quadrupole due to site K multipole.
     multipoleKPotentialAtI(mK, 2);
     quadrupoleTorque(mI, Ti);
 
     // Compute the potential due to a multipole component at site K.
     quadrupoleKPotentialAtI(mK, 3);
     double eI = multipoleEnergy(mI);
     multipoleGradient(mI, Gi);
     multipoleTorque(mI, Ti);
 
     // Need the torque on site K quadrupole due to site I multipole.
     multipoleIPotentialAtK(mI, 2);
     quadrupoleTorque(mK, Tk);
 
     double scale = c * 0.5;
     Gi[0] = scale * (Gi[0] - Gk[0]);
     Gi[1] = scale * (Gi[1] - Gk[1]);
     Gi[2] = scale * (Gi[2] - Gk[2]);
     Gk[0] = -Gi[0];
     Gk[1] = -Gi[1];
     Gk[2] = -Gi[2];
 
     Ti[0] = scale * Ti[0];
     Ti[1] = scale * Ti[1];
     Ti[2] = scale * Ti[2];
     Tk[0] = scale * Tk[0];
     Tk[1] = scale * Tk[1];
     Tk[2] = scale * Tk[2];
 
     return scale * (eK + eI);
   }
 
   /**
    * GK Permanent multipole Born grad.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @return a double.
    */
   public double multipoleEnergyBornGrad(PolarizableMultipole mI, PolarizableMultipole mK) {
     return multipoleEnergy(mI, mK);
   }
 
   /**
    * GK Polarization Energy.
    *
    * @param mI          PolarizableMultipole at site I.
    * @param mK          PolarizableMultipole at site K.
    * @param scaleEnergy This is ignored, since masking/scaling is not applied to GK interactions
    *                    (everything is intermolecular).
    * @return a double.
    */
   public double polarizationEnergy(PolarizableMultipole mI, PolarizableMultipole mK, DoubleVector scaleEnergy) {
     return polarizationEnergy(mI, mK);
   }
 
   /**
    * GK Polarization Energy.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @return a double.
    */
   public double polarizationEnergy(PolarizableMultipole mI, PolarizableMultipole mK) {
     return switch (multipoleOrder) {
       default -> {
         // Find the GK charge potential of site I at site K.
         chargeIPotentialAtK(mI, 1);
         // Energy of induced dipole K in the field of permanent charge I.
         double eK = polarizationEnergy(mK);
         // Find the GK charge potential of site K at site I.
         chargeKPotentialAtI(mK, 1);
         // Energy of induced dipole I in the field of permanent charge K.
         double eI = polarizationEnergy(mI);
         yield c * 0.5 * (eK + eI);
       }
       case DIPOLE -> {
         // Find the GK dipole potential of site I at site K.
         dipoleIPotentialAtK(mI.dx, mI.dy, mI.dz, 1);
         // Energy of induced dipole K in the field of permanent dipole I.
         double eK = polarizationEnergy(mK);
         // Find the GK induced dipole potential of site I at site K.
         dipoleIPotentialAtK(mI.ux, mI.uy, mI.uz, 2);
         // Energy of permanent multipole K in the field of induced dipole I.
         eK += 0.5 * multipoleEnergy(mK);
         // Find the GK dipole potential of site K at site I.
         dipoleKPotentialAtI(mK.dx, mK.dy, mK.dz, 1);
         // Energy of induced dipole I in the field of permanent dipole K.
         double eI = polarizationEnergy(mI);
         // Find the GK induced dipole potential of site K at site I.
         dipoleKPotentialAtI(mK.ux, mK.uy, mK.uz, 2);
         // Energy of permanent multipole I in the field of induced dipole K.
         eI += 0.5 * multipoleEnergy(mI);
         yield c * 0.5 * (eK + eI);
       }
       case QUADRUPOLE -> {
         // Find the GK quadrupole potential of site I at site K.
         quadrupoleIPotentialAtK(mI, 1);
         // Energy of induced dipole K in the field of permanent quadrupole I.
         double eK = polarizationEnergy(mK);
         // Find the GK quadrupole potential of site K at site I.
         quadrupoleKPotentialAtI(mK, 1);
         // Energy of induced dipole I in the field of permanent quadrupole K.
         double eI = polarizationEnergy(mI);
         yield c * 0.5 * (eK + eI);
       }
     };
   }
 
   /**
    * GK Polarization Energy.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @return a double.
    */
   public double polarizationEnergyBorn(PolarizableMultipole mI, PolarizableMultipole mK) {
     return switch (multipoleOrder) {
       default -> {
         // Find the GK charge potential of site I at site K.
         chargeIPotentialAtK(mI, 1);
         // Energy of induced dipole K in the field of permanent charge I.
         double eK = polarizationEnergyS(mK);
         // Find the GK charge potential of site K at site I.
         chargeKPotentialAtI(mK, 1);
         // Energy of induced dipole I in the field of permanent charge K.
         double eI = polarizationEnergyS(mI);
         yield c * 0.5 * (eK + eI);
       }
       case DIPOLE -> {
         // Find the GK dipole potential of site I at site K.
         dipoleIPotentialAtK(mI.dx, mI.dy, mI.dz, 1);
         // Energy of induced dipole K in the field of permanent dipole I.
         double eK = polarizationEnergyS(mK);
         // Find the GK induced dipole potential of site I at site K.
         dipoleIPotentialAtK(mI.sx, mI.sy, mI.sz, 2);
         // Energy of permanent multipole K in the field of induced dipole I.
         eK += 0.5 * multipoleEnergy(mK);
         // Find the GK dipole potential of site K at site I.
         dipoleKPotentialAtI(mK.dx, mK.dy, mK.dz, 1);
         // Energy of induced dipole I in the field of permanent dipole K.
         double eI = polarizationEnergyS(mI);
         // Find the GK induced dipole potential of site K at site I.
         dipoleKPotentialAtI(mK.sx, mK.sy, mK.sz, 2);
         // Energy of permanent multipole I in the field of induced dipole K.
         eI += 0.5 * multipoleEnergy(mI);
         yield c * 0.5 * (eK + eI);
       }
       case QUADRUPOLE -> {
         // Find the GK quadrupole potential of site I at site K.
         quadrupoleIPotentialAtK(mI, 1);
         // Energy of induced dipole K in the field of permanent quadrupole I.
         double eK = polarizationEnergyS(mK);
         // Find the GK quadrupole potential of site K at site I.
         quadrupoleKPotentialAtI(mK, 1);
         // Energy of induced dipole I in the field of permanent quadrupole K.
         double eI = polarizationEnergyS(mI);
         yield c * 0.5 * (eK + eI);
       }
     };
   }
 
   /**
    * Polarization Energy and Gradient.
    *
    * @param mI            PolarizableMultipole at site I.
    * @param mK            PolarizableMultipole at site K.
    * @param inductionMask This is ignored, since masking/scaling is not applied to GK
    *                      interactions (everything is intermolecular).
    * @param energyMask    This is ignored, since masking/scaling is not applied to GK interactions
    *                      (everything is intermolecular).
    * @param mutualMask    This should be set to zero for direction polarization.
    * @param Gi            an array of double values.
    * @param Ti            an array of double values.
    * @param Tk            an array of double values.
    * @return a double.
    */
   @Override
   public double polarizationEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                               double inductionMask, double energyMask, double mutualMask, double[] Gi, double[] Ti,
                                               double[] Tk) {
     return switch (multipoleOrder) {
       default -> monopolePolarizationEnergyAndGradient(mI, mK, Gi);
       case DIPOLE -> dipolePolarizationEnergyAndGradient(mI, mK, mutualMask, Gi, Ti, Tk);
       case QUADRUPOLE -> quadrupolePolarizationEnergyAndGradient(mI, mK, Gi, Ti, Tk);
     };
   }
 
   /**
    * Monopole Polarization Energy and Gradient.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @param Gi an array of double values.
    * @return a double.
    */
   public double monopolePolarizationEnergyAndGradient(PolarizableMultipole mI,
                                                       PolarizableMultipole mK, double[] Gi) {
 
     // Find the permanent multipole potential at site k.
     chargeIPotentialAtK(mI, 2);
     // Energy of induced dipole k in the field of multipole i.
     double eK = polarizationEnergy(mK);
     // Derivative with respect to moving atom k.
     Gi[0] = -(mK.sx * E200 + mK.sy * E110 + mK.sz * E101);
     Gi[1] = -(mK.sx * E110 + mK.sy * E020 + mK.sz * E011);
     Gi[2] = -(mK.sx * E101 + mK.sy * E011 + mK.sz * E002);
 
     // Find the permanent multipole potential and derivatives at site i.
     chargeKPotentialAtI(mK, 2);
     // Energy of induced dipole i in the field of multipole k.
     double eI = polarizationEnergy(mI);
     // Derivative with respect to moving atom i.
     Gi[0] += (mI.sx * E200 + mI.sy * E110 + mI.sz * E101);
     Gi[1] += (mI.sx * E110 + mI.sy * E020 + mI.sz * E011);
     Gi[2] += (mI.sx * E101 + mI.sy * E011 + mI.sz * E002);
 
     double scale = c * 0.5;
     Gi[0] *= scale;
     Gi[1] *= scale;
     Gi[2] *= scale;
 
     // Total polarization energy.
     return scale * (eI + eK);
   }
 
   /**
    * Dipole Polarization Energy and Gradient.
    *
    * @param mI         PolarizableMultipole at site I.
    * @param mK         PolarizableMultipole at site K.
    * @param mutualMask This should be set to zero for direction polarization.
    * @param Gi         an array of double values.
    * @param Ti         an array of double values.
    * @param Tk         an array of double values.
    * @return a double.
    */
   public double dipolePolarizationEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                                     double mutualMask, double[] Gi, double[] Ti, double[] Tk) {
 
     // Find the permanent multipole potential and derivatives at site k.
     dipoleIPotentialAtK(mI.dx, mI.dy, mI.dz, 2);
     // Energy of induced dipole k in the field of multipole i.
     double eK = polarizationEnergy(mK);
     // Derivative with respect to moving atom k.
     Gi[0] = -(mK.sx * E200 + mK.sy * E110 + mK.sz * E101);
     Gi[1] = -(mK.sx * E110 + mK.sy * E020 + mK.sz * E011);
     Gi[2] = -(mK.sx * E101 + mK.sy * E011 + mK.sz * E002);
     // Find the potential at K due to the averaged induced dipole at site i.
     dipoleIPotentialAtK(mI.sx, mI.sy, mI.sz, 2);
     dipoleTorque(mK, Tk);
 
     // Find the GK induced dipole potential of site I at site K.
     dipoleIPotentialAtK(mI.ux, mI.uy, mI.uz, 3);
     // Energy of permanent multipole K in the field of induced dipole I.
     eK += 0.5 * multipoleEnergy(mK);
     double[] G = new double[3];
     multipoleGradient(mK, G);
     Gi[0] -= G[0];
     Gi[1] -= G[1];
     Gi[2] -= G[2];
     multipoleTorque(mK, Tk);
 
     // Find the permanent multipole potential and derivatives at site i.
     dipoleKPotentialAtI(mK.dx, mK.dy, mK.dz, 2);
     // Energy of induced dipole i in the field of multipole k.
     double eI = polarizationEnergy(mI);
     // Derivative with respect to moving atom i.
     Gi[0] += (mI.sx * E200 + mI.sy * E110 + mI.sz * E101);
     Gi[1] += (mI.sx * E110 + mI.sy * E020 + mI.sz * E011);
     Gi[2] += (mI.sx * E101 + mI.sy * E011 + mI.sz * E002);
     // Find the potential at I due to the averaged induced dipole at k.
     dipoleKPotentialAtI(mK.sx, mK.sy, mK.sz, 2);
     dipoleTorque(mI, Ti);
 
     // Find the GK induced dipole potential of site K at site I.
     dipoleKPotentialAtI(mK.ux, mK.uy, mK.uz, 3);
     // Energy of permanent multipole I in the field of induced dipole K.
     eI += 0.5 * multipoleEnergy(mI);
     G = new double[3];
     multipoleGradient(mI, G);
     Gi[0] += G[0];
     Gi[1] += G[1];
     Gi[2] += G[2];
     multipoleTorque(mI, Ti);
 
     // Get the induced-induced portion of the force (Ud . dC/dX . Up).
     // This contribution does not exist for direct polarization (mutualMask == 0.0).
     if (mutualMask != 0.0) {
       // Find the potential and its derivatives at k due to induced dipole i.
       dipoleIPotentialAtK(mI.ux, mI.uy, mI.uz, 2);
       Gi[0] -= mutualMask * (mK.px * E200 + mK.py * E110 + mK.pz * E101);
       Gi[1] -= mutualMask * (mK.px * E110 + mK.py * E020 + mK.pz * E011);
       Gi[2] -= mutualMask * (mK.px * E101 + mK.py * E011 + mK.pz * E002);
 
       // Find the potential and its derivatives at i due to induced dipole k.
       dipoleKPotentialAtI(mK.ux, mK.uy, mK.uz, 2);
       Gi[0] += mutualMask * (mI.px * E200 + mI.py * E110 + mI.pz * E101);
       Gi[1] += mutualMask * (mI.px * E110 + mI.py * E020 + mI.pz * E011);
       Gi[2] += mutualMask * (mI.px * E101 + mI.py * E011 + mI.pz * E002);
     }
 
     // Total polarization energy.
     double scale = c * 0.5;
     double energy = scale * (eI + eK);
     Gi[0] *= scale;
     Gi[1] *= scale;
     Gi[2] *= scale;
     Ti[0] *= scale;
     Ti[1] *= scale;
     Ti[2] *= scale;
     Tk[0] *= scale;
     Tk[1] *= scale;
     Tk[2] *= scale;
 
     return energy;
   }
 
   /**
    * Quadrupole Polarization Energy and Gradient.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @param Gi an array of double values.
    * @param Ti an array of double values.
    * @param Tk an array of double values.
    * @return a double.
    */
   public double quadrupolePolarizationEnergyAndGradient(PolarizableMultipole mI, PolarizableMultipole mK,
                                                         double[] Gi, double[] Ti, double[] Tk) {
 
     // Find the permanent multipole potential and derivatives at site k.
     quadrupoleIPotentialAtK(mI, 2);
     // Energy of induced dipole k in the field of multipole i.
     double eK = polarizationEnergy(mK);
     // Derivative with respect to moving atom k.
     Gi[0] = -(mK.sx * E200 + mK.sy * E110 + mK.sz * E101);
     Gi[1] = -(mK.sx * E110 + mK.sy * E020 + mK.sz * E011);
     Gi[2] = -(mK.sx * E101 + mK.sy * E011 + mK.sz * E002);
 
     // Find the permanent multipole potential and derivatives at site i.
     quadrupoleKPotentialAtI(mK, 2);
     // Energy of induced dipole i in the field of multipole k.
     double eI = polarizationEnergy(mI);
     // Derivative with respect to moving atom i.
     Gi[0] += (mI.sx * E200 + mI.sy * E110 + mI.sz * E101);
     Gi[1] += (mI.sx * E110 + mI.sy * E020 + mI.sz * E011);
     Gi[2] += (mI.sx * E101 + mI.sy * E011 + mI.sz * E002);
 
     double scale = c * 0.5;
     Gi[0] *= scale;
     Gi[1] *= scale;
     Gi[2] *= scale;
 
     // Find the potential and its derivatives at K due to the averaged induced dipole at site i.
     dipoleIPotentialAtK(scale * mI.sx, scale * mI.sy, scale * mI.sz, 2);
     quadrupoleTorque(mK, Tk);
 
     // Find the potential and its derivatives at I due to the averaged induced dipole at k.
     dipoleKPotentialAtI(scale * mK.sx, scale * mK.sy, scale * mK.sz, 2);
     quadrupoleTorque(mI, Ti);
 
     // Total polarization energy.
     return scale * (eI + eK);
   }
 
   /**
    * GK Direct Polarization Born grad.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @return Partial derivative of the Polarization energy with respect to a Born grad.
    */
   public double polarizationEnergyBornGrad(PolarizableMultipole mI, PolarizableMultipole mK) {
     return 2.0 * polarizationEnergyBorn(mI, mK);
   }
 
   /**
    * GK Mutual Polarization Contribution to the Born grad.
    *
    * @param mI PolarizableMultipole at site I.
    * @param mK PolarizableMultipole at site K.
    * @return Mutual Polarization contribution to the partial derivative with respect to a Born grad.
    */
   public double mutualPolarizationEnergyBornGrad(PolarizableMultipole mI, PolarizableMultipole mK) {
     double db = 0.0;
     if (multipoleOrder == GKMultipoleOrder.DIPOLE) {
       // Find the potential and its derivatives at k due to induced dipole i.
       dipoleIPotentialAtK(mI.ux, mI.uy, mI.uz, 2);
       db = 0.5 * (mK.px * E100 + mK.py * E010 + mK.pz * E001);
 
       // Find the potential and its derivatives at i due to induced dipole k.
       dipoleKPotentialAtI(mK.ux, mK.uy, mK.uz, 2);
       db += 0.5 * (mI.px * E100 + mI.py * E010 + mI.pz * E001);
     }
     return c * db;
   }
 
   /**
    * Generate source terms for the Kirkwood version of the Challacombe et al. recursion.
    */
   @Override
   protected void source(double[] work) {
     gkSource.source(work, multipoleOrder);
   }
 
 }