Merge pull request #2714 from julient31/STT2

Improvement of the SPIN package by removing a limitation

doc/src/fix_precession_spin.rst

@@ -12,7 +12,7 @@ Syntax
 
 * ID, group are documented in :doc:`fix <fix>` command
 * precession/spin = style name of this fix command
-* style = *zeeman* or *anisotropy* or *cubic*
+* style = *zeeman* or *anisotropy* or *cubic* or *stt*
 
   .. parsed-literal::
 
@@ -22,12 +22,12 @@ Syntax
        *anisotropy* args = K x y z
          K = intensity of the magnetic anisotropy (in eV)
          x y z = vector direction of the anisotropy
-
-  .. parsed-literal::
-
        *cubic* args = K1 K2c n1x n1y n1x n2x n2y n2z n3x n3y n3z
          K1 and K2c = intensity of the magnetic anisotropy (in eV)
          n1x to n3z = three direction vectors of the cubic anisotropy
+       *stt* args = J x y z
+         J = intensity of the spin-transfer torque field
+         x y z = vector direction of the field
 
 Examples
 """"""""
@@ -125,6 +125,11 @@ axis along the :math:`(1 1 1)`-type cube diagonals).  :math:`K_2^c >
 diagonals.  See chapter 2 of :ref:`(Skomski) <Skomski1>` for more
 details on cubic anisotropies.
 
+Style *stt* is used to simulate the interaction between the spins and 
+a spin-transfer torque.
+See equation (7) of :ref:`(Chirac) <Chirac1>` for more details about the 
+implemented spin-transfer torque term. 
+
 In all cases, the choice of :math:`(x y z)` only imposes the vector
 directions for the forces. Only the direction of the vector is
 important; its length is ignored (the entered vectors are
@@ -132,6 +137,16 @@ normalized).
 
 Those styles can be combined within one single command line.
 
+.. note::
+
+   The norm of all vectors defined with the precession/spin command 
+   have to be non-zero. For example, defining
+   "fix 1 all precession/spin zeeman 0.1 0.0 0.0 0.0" would result
+   in an error message. 
+   Since those vector components are used to compute the inverse of the
+   field (or anisotropy) vector norm, setting a zero-vector would result 
+   in a division by zero.
+
 ----------
 
 Restart, fix_modify, output, run start/stop, minimize info
@@ -162,11 +177,6 @@ is only enabled if LAMMPS was built with this package, and if the
 atom_style "spin" was declared.  See the :doc:`Build package
 <Build_package>` doc page for more info.
 
-The *precession/spin* style can only be declared once. If more than
-one precession type (for example combining an anisotropy and a Zeeman
-interactions) has to be declared, they have to be chained in the same
-command line (as shown in the examples above).
-
 Related commands
 """"""""""""""""
 
@@ -184,3 +194,9 @@ none
 
 **(Skomski)** Skomski, R. (2008). Simple models of magnetism.
 Oxford University Press.
+
+.. _Chirac1:
+
+**(Chirac)** Chirac, Théophile, et al.  Ultrafast antiferromagnetic 
+switching in NiO induced by spin transfer torques. 
+Physical Review B 102.13 (2020): 134415.

examples/SPIN/test_problems/README

@@ -45,3 +45,14 @@ directory.
   results (computed by the python script).
   Note: This example is a reworked version of a test problem 
   provided by Martin Kroger (ETHZ).  
+
+- validation_nve:
+  simulates a small assembly of magnetic atoms (54). The atoms are 
+  coupled by an exchange interaction and a mechanical potential 
+  (EAM here). 
+  This example represents an NVE run: the total energy of the 
+  system is preserved, whereas the spin and lattice energy 
+  reservoirs are exchanging energy.
+  Run as: ./run-test-nve.sh
+  Output: res_lammps.dat contains the data. The results are displayed
+  by nve_spin_lattice.pdf. 

examples/SPIN/test_problems/run_all.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+# test 1: damping and exchange 
+cd validation_damped_exchange/
+./run-test-exchange.sh
+rm dump.data res_lammps.dat res_llg.dat
+cd ..
+
+# test 2: damping and Zeeman
+cd validation_damped_precession/
+./run-test-prec.sh
+rm res_lammps.dat res_llg.dat
+cd ..
+
+# test 3: langevin, damping and Zeeman
+cd validation_langevin_precession/
+./run-test-prec.sh
+rm average_spin test-prec-spin.in res_lammps.dat res_langevin.dat
+cd ..
+
+# test 4: NVE run, test Etot preservation
+cd validation_nve/
+./run-test-nve.sh
+rm nve_spin_lattice.pdf res_lammps.dat
+cd ..

examples/SPIN/test_problems/validation_nvt/in.spin.nvt_lattice

@@ -31,8 +31,8 @@ neighbor 	0.1 bin
 neigh_modify 	every 10 check yes delay 20
 
 fix 		1 all precession/spin zeeman 0.0 0.0 0.0 1.0
-fix             2 all langevin 200.0 200.0 1.0 48279
+fix             2 all langevin 200.0 200.0 0.1 48279
-fix 		3 all langevin/spin 0.0 0.00001 321
+fix 		3 all langevin/spin 0.0 0.0 321
 fix 		4 all nve/spin lattice moving
 timestep	0.001
 

examples/SPIN/test_problems/validation_nvt/in.spin.nvt_spin

@@ -22,16 +22,19 @@ mass		1 55.845
 set 		group all spin 2.2 0.0 0.0 1.0
 velocity 	all create 0 4928459 rot yes dist gaussian
 
-pair_style 	hybrid/overlay eam/alloy spin/exchange 3.5
+# pair_style 	hybrid/overlay eam/alloy spin/exchange 3.5
+pair_style 	hybrid/overlay eam/alloy spin/exchange 4.0 spin/neel 4.0
 pair_coeff 	* * eam/alloy Fe_Mishin2006.eam.alloy Fe
 pair_coeff 	* * spin/exchange exchange 3.4 0.1 0.2171 1.841
+pair_coeff 	* * spin/neel neel 4.0 0.02 0.0 1.841 0.0 0.0 1.0 
 
 neighbor 	0.1 bin
 neigh_modify 	every 10 check yes delay 20
 
 fix 		1 all precession/spin zeeman 0.0 0.0 0.0 1.0
-fix 		2 all langevin/spin 200.0 0.01 321
+fix             2 all langevin 0.0 0.0 0.0 48279
-fix 		3 all nve/spin lattice moving
+fix 		3 all langevin/spin 200.0 0.01 321
+fix 		4 all nve/spin lattice moving
 timestep	0.001
 
 # compute and output options

src/SPIN/fix_langevin_spin.cpp

@@ -24,9 +24,11 @@
 #include "fix_langevin_spin.h"
 #include <cmath>
 #include <cstring>
+#include "atom.h"
 #include "comm.h"
 #include "error.h"
 #include "force.h"
+#include "group.h"
 #include "math_const.h"
 #include "memory.h"
 #include "modify.h"
@@ -42,7 +44,7 @@ using namespace MathConst;
 /* ---------------------------------------------------------------------- */
 
 FixLangevinSpin::FixLangevinSpin(LAMMPS *lmp, int narg, char **arg) :
-  Fix(lmp, narg, arg), id_temp(nullptr), random(nullptr)
+  Fix(lmp, narg, arg), random(nullptr)
 {
   if (narg != 6) error->all(FLERR,"Illegal langevin/spin command");
 
@@ -106,10 +108,8 @@ void FixLangevinSpin::init()
 
   double hbar = force->hplanck/MY_2PI;  // eV/(rad.THz)
   double kb = force->boltz;             // eV/K
-  // D = (MY_2PI*alpha_t*gil_factor*kb*temp);
 
   D = (alpha_t*gil_factor*kb*temp);
-  // D = (12.0/MY_2PI)*(MY_2PI*alpha_t*gil_factor*kb*temp);
   D /= (hbar*dts);
   sigma = sqrt(2.0*D);
 }
@@ -142,7 +142,7 @@ void FixLangevinSpin::add_tdamping(double spi[3], double fmi[3])
 
 /* ---------------------------------------------------------------------- */
 
-void FixLangevinSpin::add_temperature(double fmi[3])
+void FixLangevinSpin::add_temperature(int i, double spi[3], double fmi[3])
 {
   // double rx = sigma*(2.0*random->uniform() - 1.0);
   // double ry = sigma*(2.0*random->uniform() - 1.0);
@@ -150,6 +150,7 @@ void FixLangevinSpin::add_temperature(double fmi[3])
   double rx = sigma*random->gaussian();
   double ry = sigma*random->gaussian();
   double rz = sigma*random->gaussian();
+  double hbar = force->hplanck/MY_2PI;
 
   // adding the random field
 
@@ -163,3 +164,14 @@ void FixLangevinSpin::add_temperature(double fmi[3])
   fmi[1] *= gil_factor;
   fmi[2] *= gil_factor;
 }
+
+/* ---------------------------------------------------------------------- */
+
+void FixLangevinSpin::compute_single_langevin(int i, double spi[3], double fmi[3])
+{
+  int *mask = atom->mask;
+  if (mask[i] & groupbit) {
+    if (tdamp_flag) add_tdamping(spi,fmi);
+    if (temp_flag) add_temperature(i,spi,fmi);
+  }
+}

src/SPIN/fix_langevin_spin.h

@@ -26,7 +26,7 @@ namespace LAMMPS_NS {
 
 class FixLangevinSpin : public Fix {
  public:
-  int tdamp_flag,ldamp_flag,temp_flag;           // damping and temperature flags
+  int tdamp_flag,temp_flag;     // damping and temperature flags
 
   FixLangevinSpin(class LAMMPS *, int, char **);
   virtual ~FixLangevinSpin();
@@ -34,7 +34,8 @@ class FixLangevinSpin : public Fix {
   void init();
   void setup(int);
   void add_tdamping(double *, double *);                 // add transverse damping
-  void add_temperature(double *);                        // add temperature
+  void add_temperature(int, double *, double *); 
+  void compute_single_langevin(int, double *, double *);
 
  protected:
   double alpha_t;               // transverse mag. damping
@@ -43,9 +44,6 @@ class FixLangevinSpin : public Fix {
   double D,sigma;               // bath intensity var.
   double gil_factor;            // gilbert's prefactor
 
-  char *id_temp;
-  class Compute *temperature;
-
   int nlevels_respa;
   class RanMars *random;
   int seed;

src/SPIN/fix_nve_spin.cpp

@@ -60,7 +60,8 @@ enum{NONE};
 
 FixNVESpin::FixNVESpin(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  pair(nullptr), spin_pairs(nullptr),
+  pair(nullptr), spin_pairs(nullptr), locklangevinspin(nullptr), 
+  locksetforcespin(nullptr), lockprecessionspin(nullptr),
   rsec(nullptr), stack_head(nullptr), stack_foot(nullptr),
   backward_stacks(nullptr), forward_stacks(nullptr)
 {
@@ -75,6 +76,9 @@ FixNVESpin::FixNVESpin(LAMMPS *lmp, int narg, char **arg) :
   npairs = 0;
   npairspin = 0;
   
+  // test nprec
+  nprecspin = nlangspin = nsetspin = 0;
+
   // checking if map array or hash is defined
 
   if (atom->map_style == Atom::MAP_NONE)
@@ -125,7 +129,6 @@ FixNVESpin::FixNVESpin(LAMMPS *lmp, int narg, char **arg) :
   maglangevin_flag = 0;
   tdamp_flag = temp_flag = 0;
   setforce_spin_flag = 0;
-
 }
 
 /* ---------------------------------------------------------------------- */
@@ -138,6 +141,8 @@ FixNVESpin::~FixNVESpin()
   memory->destroy(forward_stacks);
   memory->destroy(backward_stacks);
   delete [] spin_pairs;
+  delete [] locklangevinspin;
+  delete [] lockprecessionspin;
 }
 
 /* ---------------------------------------------------------------------- */
@@ -189,20 +194,20 @@ void FixNVESpin::init()
 
   // loop 2: fill vector with ptrs to Pair/Spin styles
 
-  int count = 0;
+  int count1 = 0;
   if (npairspin == 1) {
-    count = 1;
+    count1 = 1;
     spin_pairs[0] = (PairSpin *) force->pair_match("spin",0,0);
   } else if (npairspin > 1) {
     for (int i = 0; i<npairs; i++) {
       if (force->pair_match("spin",0,i)) {
-        spin_pairs[count] = (PairSpin *) force->pair_match("spin",0,i);
+        spin_pairs[count1] = (PairSpin *) force->pair_match("spin",0,i);
-        count++;
+        count1++;
       }
     }
   }
 
-  if (count != npairspin)
+  if (count1 != npairspin)
     error->all(FLERR,"Incorrect number of spin pairs");
 
   // set pair/spin and long/spin flags
@@ -215,30 +220,71 @@ void FixNVESpin::init()
     }
   }
 
-  // ptrs FixPrecessionSpin classes
+  // set ptrs for fix precession/spin styles
+  
+  // loop 1: obtain # of fix precession/spin styles
   
   int iforce;
   for (iforce = 0; iforce < modify->nfix; iforce++) {
     if (strstr(modify->fix[iforce]->style,"precession/spin")) {
-      precession_spin_flag = 1;
+      nprecspin++;
-      lockprecessionspin = (FixPrecessionSpin *) modify->fix[iforce];
     }
   }
   
-  // ptrs on the FixLangevinSpin class
+  // init length of vector of ptrs to precession/spin styles
+
+  if (nprecspin > 0) {
+    lockprecessionspin = new FixPrecessionSpin*[nprecspin];
+  }
+  
+  // loop 2: fill vector with ptrs to precession/spin styles
+
+  int count2 = 0;
+  if (nprecspin > 0) {
+    for (iforce = 0; iforce < modify->nfix; iforce++) {
+      if (strstr(modify->fix[iforce]->style,"precession/spin")) {
+        precession_spin_flag = 1;
+        lockprecessionspin[count2] = (FixPrecessionSpin *) modify->fix[iforce];
+        count2++;
+      }
+    }
+  }
+  
+  if (count2 != nprecspin)
+    error->all(FLERR,"Incorrect number of fix precession/spin");
+
+  // set ptrs for fix langevin/spin styles
+  
+  // loop 1: obtain # of fix langevin/spin styles
   
   for (iforce = 0; iforce < modify->nfix; iforce++) {
     if (strstr(modify->fix[iforce]->style,"langevin/spin")) {
-      maglangevin_flag = 1;
+      nlangspin++;
-      locklangevinspin = (FixLangevinSpin *) modify->fix[iforce];
     }
   }
   
-  if (maglangevin_flag) {
+  // init length of vector of ptrs to precession/spin styles
-   if (locklangevinspin->tdamp_flag == 1) tdamp_flag = 1;
+
-   if (locklangevinspin->temp_flag == 1) temp_flag = 1;
+  if (nlangspin > 0) {
+    locklangevinspin = new FixLangevinSpin*[nprecspin];
   }
   
+  // loop 2: fill vector with ptrs to precession/spin styles
+
+  count2 = 0;
+  if (nlangspin > 0) {
+    for (iforce = 0; iforce < modify->nfix; iforce++) {
+      if (strstr(modify->fix[iforce]->style,"langevin/spin")) {
+        maglangevin_flag = 1;
+        locklangevinspin[count2] = (FixLangevinSpin *) modify->fix[iforce];
+        count2++;
+      }
+    }
+  }
+  
+  if (count2 != nlangspin)
+    error->all(FLERR,"Incorrect number of fix precession/spin");
+  
   // ptrs FixSetForceSpin classes
 
   for (iforce = 0; iforce < modify->nfix; iforce++) {
@@ -471,17 +517,16 @@ void FixNVESpin::ComputeInteractionsSpin(int i)
   // update magnetic precession interactions
 
   if (precession_spin_flag) {
-    lockprecessionspin->compute_single_precession(i,spi,fmi);
+    for (int k = 0; k < nprecspin; k++) {
+      lockprecessionspin[k]->compute_single_precession(i,spi,fmi);
+    }
   }
 
   // update langevin damping and random force
 
   if (maglangevin_flag) {               // mag. langevin
-    if (tdamp_flag) {                   // transverse damping
+    for (int k = 0; k < nlangspin; k++) {
-      locklangevinspin->add_tdamping(spi,fmi);
+      locklangevinspin[k]->compute_single_langevin(i,spi,fmi);
-    }
-    if (temp_flag) {                    // spin temperature
-      locklangevinspin->add_temperature(fmi);
     }
   }
 
@@ -496,7 +541,6 @@ void FixNVESpin::ComputeInteractionsSpin(int i)
   fm[i][0] = fmi[0];
   fm[i][1] = fmi[1];
   fm[i][2] = fmi[2];
-
 }
 
 /* ----------------------------------------------------------------------

src/SPIN/fix_nve_spin.h

@@ -61,11 +61,20 @@ friend class PairSpin;
   int tdamp_flag, temp_flag;
   int setforce_spin_flag;
 
-  // pointers to magnetic fixes
+  // pointers to fix langevin/spin styles
 
-  class FixPrecessionSpin *lockprecessionspin;
+  int nlangspin;
-  class FixLangevinSpin *locklangevinspin;
+  class FixLangevinSpin **locklangevinspin;
-  class FixSetForceSpin *locksetforcespin;
+  
+  // pointers to fix setforce/spin styles
+  
+  int nsetspin;
+  class FixSetForceSpin *locksetforcespin; // to be done
+  
+  // pointers to fix precession/spin styles
+
+  int nprecspin; 
+  class FixPrecessionSpin **lockprecessionspin;
 
   // pointers to magnetic pair styles
 

src/SPIN/fix_precession_spin.cpp

@@ -67,6 +67,9 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
   H_field = 0.0;
   nhx = nhy = nhz = 0.0;
   hx = hy = hz = 0.0;
+  stt_field = 0.0;
+  nsttx = nstty = nsttz = 0.0;
+  sttx = stty = sttz = 0.0;
   Ka = 0.0;
   nax = nay = naz = 0.0;
   Kax = Kay = Kaz = 0.0;
@@ -74,8 +77,11 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
   nc1x = nc1y = nc1z = 0.0;
   nc2x = nc2y = nc2z = 0.0;
   nc3x = nc3y = nc3z = 0.0;
+  K6 = 0.0;
+  n6x = n6y = n6z = 0.0;
+  m6x = m6y = m6z = 0.0;
 
-  zeeman_flag = aniso_flag = cubic_flag = 0;
+  zeeman_flag = stt_flag = aniso_flag = cubic_flag = hexaniso_flag =  0;
 
   int iarg = 3;
   while (iarg < narg) {
@@ -87,6 +93,14 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
       nhy = utils::numeric(FLERR,arg[iarg+3],false,lmp);
       nhz = utils::numeric(FLERR,arg[iarg+4],false,lmp);
       iarg += 5;
+    } else if (strcmp(arg[iarg],"stt") == 0) {
+      if (iarg+4 > narg) error->all(FLERR,"Illegal fix precession/spin command");
+      stt_flag = 1;
+      stt_field = utils::numeric(FLERR,arg[iarg+1],false,lmp);
+      nsttx = utils::numeric(FLERR,arg[iarg+2],false,lmp);
+      nstty = utils::numeric(FLERR,arg[iarg+3],false,lmp);
+      nsttz = utils::numeric(FLERR,arg[iarg+4],false,lmp);
+      iarg += 5;
     } else if (strcmp(arg[iarg],"anisotropy") == 0) {
       if (iarg+4 > narg) error->all(FLERR,"Illegal fix precession/spin command");
       aniso_flag = 1;
@@ -110,41 +124,111 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
       nc3y = utils::numeric(FLERR,arg[iarg+10],false,lmp);
       nc3z = utils::numeric(FLERR,arg[iarg+11],false,lmp);
       iarg += 12;
+    } else if (strcmp(arg[iarg],"hexaniso") == 0) {
+      if (iarg+7 > narg) error->all(FLERR,"Illegal fix precession/spin command");
+      hexaniso_flag = 1;
+      K6 = utils::numeric(FLERR,arg[iarg+1],false,lmp);
+      n6x = utils::numeric(FLERR,arg[iarg+2],false,lmp);
+      n6y = utils::numeric(FLERR,arg[iarg+3],false,lmp);
+      n6z = utils::numeric(FLERR,arg[iarg+4],false,lmp);
+      m6x = utils::numeric(FLERR,arg[iarg+5],false,lmp);
+      m6y = utils::numeric(FLERR,arg[iarg+6],false,lmp);
+      m6z = utils::numeric(FLERR,arg[iarg+7],false,lmp);
+      iarg += 8;
     } else error->all(FLERR,"Illegal precession/spin command");
   }
 
   // normalize vectors
 
-  double inorm;
+  double norm2,inorm;
   if (zeeman_flag) {
-    inorm = 1.0/sqrt(nhx*nhx + nhy*nhy + nhz*nhz);
+    norm2 = nhx*nhx + nhy*nhy + nhz*nhz;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
     nhx *= inorm;
     nhy *= inorm;
     nhz *= inorm;
   }
 
+  if (stt_flag) {
+    norm2 = nsttx*nsttx + nstty*nstty + nsttz*nsttz;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
+    nsttx *= inorm;
+    nstty *= inorm;
+    nsttz *= inorm;
+  }
+
   if (aniso_flag) {
-    inorm = 1.0/sqrt(nax*nax + nay*nay + naz*naz);
+    norm2 = nax*nax + nay*nay + naz*naz;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
     nax *= inorm;
     nay *= inorm;
     naz *= inorm;
   }
 
   if (cubic_flag) {
-    inorm = 1.0/sqrt(nc1x*nc1x + nc1y*nc1y + nc1z*nc1z);
+    norm2 = nc1x*nc1x + nc1y*nc1y + nc1z*nc1z;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
     nc1x *= inorm;
     nc1y *= inorm;
     nc1z *= inorm;
-    inorm = 1.0/sqrt(nc2x*nc2x + nc2y*nc2y + nc2z*nc2z);
+    
+    norm2 = nc2x*nc2x + nc2y*nc2y + nc2z*nc2z;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
     nc2x *= inorm;
     nc2y *= inorm;
     nc2z *= inorm;
-    inorm = 1.0/sqrt(nc3x*nc3x + nc3y*nc3y + nc3z*nc3z);
+    
+    norm2 = nc3x*nc3x + nc3y*nc3y + nc3z*nc3z;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
     nc3x *= inorm;
     nc3y *= inorm;
     nc3z *= inorm;
   }
   
+  if (hexaniso_flag) {
+    norm2 = n6x*n6x + n6y*n6y + n6z*n6z;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
+    n6x *= inorm;
+    n6y *= inorm;
+    n6z *= inorm;
+    
+    norm2 = m6x*m6x + m6y*m6y + m6z*m6z;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
+    m6x *= inorm;
+    m6y *= inorm;
+    m6z *= inorm;
+    l6x = (n6z*m6y-n6y*m6z);
+    l6y = (n6x*m6z-n6z*m6x);
+    l6z = (n6y*m6x-n6x*m6y);
+    
+    norm2 = l6x*l6x + l6y*l6y + l6z*l6z;
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
+    inorm = 1.0/sqrt(norm2);
+    l6x *= inorm;
+    l6y *= inorm;
+    l6z *= inorm;
+    m6x = (l6z*n6y-l6y*n6z);
+    m6y = (l6x*n6z-l6z*n6x);
+    m6z = (l6y*n6x-l6x*n6y);
+  }
+
   degree2rad = MY_PI/180.0;
   time_origin = update->ntimestep;
 
@@ -185,6 +269,7 @@ void FixPrecessionSpin::init()
   Kah = Ka/hbar;
   k1ch = k1c/hbar;
   k2ch = k2c/hbar;
+  K6h = K6/hbar;
 
   if (utils::strmatch(update->integrate_style,"^respa")) {
     ilevel_respa = ((Respa *) update->integrate)->nlevels-1;
@@ -199,15 +284,6 @@ void FixPrecessionSpin::init()
         error->all(FLERR,"Illegal precession/spin command");
   }
 
-  // check that fix precession/spin is only declared once
-
-  int iprec = 0;
-  for (int iforce = 0; iforce < modify->nfix; iforce++)
-    if (strstr(modify->fix[iforce]->style,"precession/spin")) iprec++;
-  if (iprec > 1)
-    error->all(FLERR,"precession/spin command can only be declared once");
-
-
   varflag = CONSTANT;
   if (magfieldstyle != CONSTANT) varflag = EQUAL;
 
@@ -283,6 +359,11 @@ void FixPrecessionSpin::post_force(int /* vflag */)
         epreci -= compute_zeeman_energy(spi);
       }
 
+      if (stt_flag) {             // compute Spin Transfer Torque
+        compute_stt(spi,fmi);
+        epreci -= compute_stt_energy(spi);
+      }
+
       if (aniso_flag) {           // compute magnetic anisotropy
         compute_anisotropy(spi,fmi);
         epreci -= compute_anisotropy_energy(spi);
@@ -293,6 +374,11 @@ void FixPrecessionSpin::post_force(int /* vflag */)
         epreci -= compute_cubic_energy(spi);
       }
 
+      if (hexaniso_flag) {        // compute hexagonal anisotropy
+        compute_hexaniso(spi,fmi);
+        epreci -= compute_hexaniso_energy(spi);
+      }
+
       emag[i] += epreci;
       eprec += epreci;
       fm[i][0] += fmi[0];
@@ -309,12 +395,16 @@ void FixPrecessionSpin::compute_single_precession(int i, double spi[3], double f
   int *mask = atom->mask;
   if (mask[i] & groupbit) {
     if (zeeman_flag) compute_zeeman(i,fmi);
+    if (stt_flag) compute_stt(spi,fmi);
     if (aniso_flag) compute_anisotropy(spi,fmi);
     if (cubic_flag) compute_cubic(spi,fmi);
+    if (hexaniso_flag) compute_hexaniso(spi,fmi);
   }
 }
 
-/* ---------------------------------------------------------------------- */
+/* ----------------------------------------------------------------------
+   Zeeman
+------------------------------------------------------------------------- */
 
 void FixPrecessionSpin::compute_zeeman(int i, double fmi[3])
 {
@@ -334,8 +424,32 @@ double FixPrecessionSpin::compute_zeeman_energy(double spi[4])
   return energy;
 }
 
+/* ----------------------------------------------------------------------
+   STT
+------------------------------------------------------------------------- */
+
+void FixPrecessionSpin::compute_stt(double spi[3], double fmi[3])
+{
+  double sx = spi[0];
+  double sy = spi[1];
+  double sz = spi[2];
+  fmi[0] += 1.0*stt_field*( sy*nsttz-sz*nstty);
+  fmi[1] += 1.0*stt_field*(-sx*nsttz+sz*nsttx);
+  fmi[2] += 1.0*stt_field*( sx*nstty-sy*nsttx);
+}
+
 /* ---------------------------------------------------------------------- */
 
+double FixPrecessionSpin::compute_stt_energy(double spi[3])
+{
+  double energy = 0.0;  // Non-conservative force
+  return energy;
+}
+
+/* ----------------------------------------------------------------------
+   compute uniaxial anisotropy interaction for spin i
+------------------------------------------------------------------------- */
+
 void FixPrecessionSpin::compute_anisotropy(double spi[3], double fmi[3])
 {
   double scalar = nax*spi[0] + nay*spi[1] + naz*spi[2];
@@ -393,9 +507,7 @@ void FixPrecessionSpin::compute_cubic(double spi[3], double fmi[3])
   fmi[2] += (fourz + sixz);
 }
 
-/* ----------------------------------------------------------------------
+/* ---------------------------------------------------------------------- */
-   compute cubic aniso energy of spin i
-------------------------------------------------------------------------- */
 
 double FixPrecessionSpin::compute_cubic_energy(double spi[3])
 {
@@ -412,6 +524,62 @@ double FixPrecessionSpin::compute_cubic_energy(double spi[3])
   return energy;
 }
 
+/* ----------------------------------------------------------------------
+   compute hexagonal anisotropy interaction for spin i
+------------------------------------------------------------------------- */
+
+void FixPrecessionSpin::compute_hexaniso(double spi[3], double fmi[3])
+{
+  double s_x,s_y,s_z;
+  double pf, phi, ssint2;
+
+  // changing to the axes' frame
+  
+  s_x = l6x*spi[0]+l6y*spi[1]+l6z*spi[2];
+  s_y = m6x*spi[0]+m6y*spi[1]+m6z*spi[2];
+  s_z = n6x*spi[0]+n6y*spi[1]+n6z*spi[2];
+
+  // hexagonal anisotropy in the axes' frame
+
+  phi = atan2(s_y,s_x);
+  ssint2 = s_x*s_x + s_y*s_y;                 // s^2sin^2(theta)
+  pf = 6.0 * K6h * ssint2*ssint2*sqrt(ssint2);   // 6*K_6*s^5*sin^5(theta)
+  double fm_x =  pf*cos(5*phi);
+  double fm_y = -pf*sin(5*phi);
+  double fm_z =  0;
+
+  // back to the lab's frame
+
+  fmi[0] += fm_x*l6x+fm_y*m6x+fm_z*n6x;
+  fmi[1] += fm_x*l6y+fm_y*m6y+fm_z*n6y;
+  fmi[2] += fm_x*l6z+fm_y*m6z+fm_z*n6z;
+}
+
+/* ----------------------------------------------------------------------
+   compute hexagonal aniso energy of spin i
+------------------------------------------------------------------------- */
+
+double FixPrecessionSpin::compute_hexaniso_energy(double spi[3])
+{
+  double energy = 0.0;
+  double s_x,s_y,s_z, phi,ssint2;
+
+  // changing to the axes' frame
+
+  s_x = l6x*spi[0]+l6y*spi[1]+l6z*spi[2];
+  s_y = m6x*spi[0]+m6y*spi[1]+m6z*spi[2];
+  s_z = n6x*spi[0]+n6y*spi[1]+n6z*spi[2];
+
+  // hexagonal anisotropy in the axes' frame
+  
+  phi = atan2(s_y,s_z);
+  ssint2 = s_x*s_x + s_y*s_y;
+
+  energy = K6 * ssint2*ssint2*ssint2*cos(6*phi);
+
+  return 2.0*energy;
+}
+
 /* ---------------------------------------------------------------------- */
 
 void FixPrecessionSpin::set_magneticprecession()
@@ -421,6 +589,13 @@ void FixPrecessionSpin::set_magneticprecession()
     hy = H_field*nhy;
     hz = H_field*nhz;
   }
+  
+  if (stt_flag) {
+    sttx = stt_field*nsttx;
+    stty = stt_field*nstty;
+    sttz = stt_field*nsttz;
+  }
+  
   if (aniso_flag) {
     Kax = 2.0*Kah*nax;
     Kay = 2.0*Kah*nay;

src/SPIN/fix_precession_spin.h

@@ -39,7 +39,7 @@ class FixPrecessionSpin : public Fix {
   void min_post_force(int);
   double compute_scalar();
 
-  int zeeman_flag, aniso_flag, cubic_flag;
+  int zeeman_flag, stt_flag, aniso_flag, cubic_flag, hexaniso_flag;
   void compute_single_precession(int, double *, double *);
 
   // zeeman calculations
@@ -47,6 +47,11 @@ class FixPrecessionSpin : public Fix {
   void compute_zeeman(int, double *);
   double compute_zeeman_energy(double *);
 
+  // stt calculations
+
+  void compute_stt(double *, double *);
+  double compute_stt_energy(double *);
+
   // uniaxial aniso calculations
 
   void compute_anisotropy(double *, double *);
@@ -57,6 +62,11 @@ class FixPrecessionSpin : public Fix {
   void compute_cubic(double *, double *);
   double compute_cubic_energy(double *);
 
+  // hexagonal aniso calculations
+  
+  void compute_hexaniso(double *, double *);
+  double compute_hexaniso_energy(double *);
+
   // storing magnetic energies
 
   int nlocal_max;               // max nlocal (for list size)
@@ -83,6 +93,12 @@ class FixPrecessionSpin : public Fix {
   double nhx, nhy, nhz;
   double hx, hy, hz;            // temp. force variables
 
+  // STT intensity and direction
+
+  double stt_field;
+  double nsttx, nstty, nsttz;
+  double sttx, stty, sttz;
+
   // magnetic anisotropy intensity and direction
 
   double Ka;                    // aniso const. in eV
@@ -98,6 +114,13 @@ class FixPrecessionSpin : public Fix {
   double nc2x,nc2y,nc2z;
   double nc3x,nc3y,nc3z;
 
+  // hexagonal anisotropy
+  double K6;               // hexagonal aniso const. in eV
+  double K6h;               // hexagonal aniso const. in rad.THz
+  double n6x,n6y,n6z;       // main axis
+  double m6x,m6y,m6z;       // secondary (perpendicular) axis
+  double l6x,l6y,l6z;       // =(m x n)
+
   void set_magneticprecession();
 
 };