Adding a new test problem for large damping in the LL equation.

examples/SPIN/test_problems/README

@@ -44,7 +44,9 @@ directory.
   energy versus temperature. Comparison to the Langevin function
   results (computed by the python script).
   Note: This example is a reworked version of a test problem 
-  provided by Martin Kroger (ETHZ).  
+  provided by Martin Kroger (ETHZ). 
+  Note 2: Two versions of this test are deployed, one at low 
+  damping (0.01) and one at large damping (1.0).
 
 - validation_nve:
   simulates a small assembly of magnetic atoms (54). The atoms are 

examples/SPIN/test_problems/run_all.sh

@@ -12,13 +12,19 @@ cd validation_damped_precession/
 rm res_lammps.dat res_llg.dat
 cd ..
 
-# test 3: langevin, damping and Zeeman
+# test 3: langevin, damping and Zeeman, low damping (0.01)
-cd validation_langevin_precession/
+cd validation_langevin_precession_d0.01/
 ./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
+# test 4: langevin, damping and Zeeman, large damping (1.0)
+cd validation_langevin_precession_d1.0/
+./run-test-prec.sh
+rm average_spin test-prec-spin.in res_lammps.dat res_langevin.dat
+cd ..
+
+# test 5: NVE run, test Etot preservation
 cd validation_nve/
 ./run-test-nve.sh
 rm nve_spin_lattice.pdf res_lammps.dat

examples/SPIN/test_problems/validation_langevin_precession_d1.0/langevin.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+
+import numpy as np, pylab, tkinter
+import matplotlib.pyplot as plt
+import mpmath as mp
+
+mub=5.78901e-5          # Bohr magneton (eV/T)
+kb=8.617333262145e-5    # Boltzman constant (eV/K)
+g=2.0                   # Lande factor (adim)
+
+Hz=10.0                # mag. field (T)
+
+#Definition of the Langevin function
+def func(t):
+    return mp.coth(g*mub*Hz/(kb*t))-1.0/(g*mub*Hz/(kb*t))
+
+npoints=200
+ti=0.01
+tf=20.0
+for i in range (0,npoints): 
+    temp=ti+i*(tf-ti)/npoints
+    print('%lf %lf %lf' % (temp,func(temp),-g*mub*Hz*func(temp)))

examples/SPIN/test_problems/validation_langevin_precession_d1.0/plot_precession.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python3
+
+#Program fitting the exchange interaction
+#Model curve: Bethe-Slater function
+import numpy as np, pylab, tkinter
+import matplotlib.pyplot as plt
+from scipy.optimize import curve_fit
+from decimal import *
+import sys, string, os
+
+
+argv = sys.argv
+if len(argv) != 3:
+  print("Syntax: ./plot_precession.py res_lammps.dat res_langevin.dat")
+  sys.exit()
+
+lammps_file = sys.argv[1]
+langevin_file = sys.argv[2]
+
+T_lmp,S_lmp,E_lmp = np.loadtxt(lammps_file, skiprows=0, usecols=(0,2,3),unpack=True)
+T_lan,S_lan,E_lan = np.loadtxt(langevin_file, skiprows=0, usecols=(0,1,2),unpack=True)
+
+plt.figure()
+plt.subplot(211)
+plt.ylabel('<Sz>')
+plt.plot(T_lmp, S_lmp, 'b-', label='LAMMPS')
+plt.plot(T_lan, S_lan, 'r--', label='Langevin')
+
+plt.subplot(212)
+plt.ylabel('E (in eV)')
+plt.plot(T_lmp, E_lmp, 'b-', label='LAMMPS')
+plt.plot(T_lan, E_lan, 'r--', label='Langevin')
+
+plt.xlabel('T (in K)')
+pylab.xlim([0,20.0])
+plt.legend()
+plt.show()

examples/SPIN/test_problems/validation_langevin_precession_d1.0/run-test-prec.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+tempi=0.0
+tempf=20.0
+
+rm res_*.dat
+
+# compute Lammps 
+N=20
+for (( i=0; i<$N; i++ ))
+do
+  temp="$(echo "$tempi+$i*($tempf-$tempi)/$N" | bc -l)"
+  sed s/temperature/${temp}/g test-prec-spin.template > \
+    test-prec-spin.in
+  
+  # test standard Lammps 
+  ./../../../../src/lmp_serial -in test-prec-spin.in 
+
+  # test spin/kk with Kokkos Lammps
+  # mpirun -np 1 ../../../../src/lmp_kokkos_mpi_only \
+  #   -k on -sf kk -in test-prec-spin.in
+  
+  Hz="$(tail -n 1 average_spin | awk -F " " '{print $3}')"
+  sz="$(tail -n 1 average_spin | awk -F " " '{print $5}')"
+  en="$(tail -n 1 average_spin | awk -F " " '{print $6}')"
+  echo $temp $Hz $sz $en >> res_lammps.dat
+done
+
+# compute Langevin
+python3 langevin.py > res_langevin.dat
+
+# plot results
+python3 plot_precession.py res_lammps.dat res_langevin.dat

examples/SPIN/test_problems/validation_langevin_precession_d1.0/test-prec-spin.template

@@ -0,0 +1,48 @@
+#LAMMPS in.run 
+ 
+units           metal
+atom_style      spin
+# atom_style      spin/kk
+atom_modify     map array
+boundary        p p p
+
+# read_data     singlespin.data
+
+lattice         sc 3.0
+region          box block 0.0 1.0 0.0 1.0 0.0 1.0
+create_box      1 box
+create_atoms    1 box
+
+mass            1 1.0
+set             type 1 spin 1.0 0.0 0.0 1.0
+
+# defines a pair/style for neighbor list, but do not use it
+pair_style      spin/exchange 4.0
+pair_coeff      * * exchange 1.0 0.0 0.0 1.0
+
+group bead      type 1  
+ 
+variable        H equal 10.0
+variable        Kan equal 0.0
+variable        Temperature equal temperature
+variable        RUN equal 1000000
+
+fix             1 all nve/spin lattice no
+fix             2 all precession/spin zeeman ${H} 0.0 0.0 1.0 anisotropy ${Kan} 0.0 0.0 1.0
+fix_modify      2 energy yes
+# fix             3 all langevin/spin ${Temperature} 0.01 12345
+fix             3 all langevin/spin ${Temperature} 1.0 12345
+
+compute         compute_spin all spin
+compute         outsp all property/atom spx spy spz sp
+compute         magsz all reduce ave c_outsp[3]
+
+thermo          50000
+thermo_style    custom step time temp vol pe c_compute_spin[5] etotal  
+
+variable        magnetic_energy equal c_compute_spin[5]
+
+fix             avespin all ave/time 1 ${RUN} ${RUN} v_Temperature v_H v_Kan c_magsz v_magnetic_energy file average_spin
+
+timestep        0.1
+run             ${RUN}