Configuration: Fix for antiparallel spins in Homogeneous_Rotation.

If two images had antiparallel spins, eg +z and -z, the resulting interpolated spins were not of unit length. This was due to the definition of the rotation axis as s1xs2, resulting in the null vector. The new behaviour is that either ex or ey is chosen as rotation axis and a warning is logged.

core/src/utility/Configuration_Chain.cpp

@@ -1,6 +1,7 @@
 #include <utility/Configuration_Chain.hpp>
 #include <utility/Configurations.hpp>
 #include <utility/Constants.hpp>
+#include <utility/Logging.hpp>
 #include <data/Spin_System.hpp>
 #include <engine/Vectormath.hpp>
 
@@ -31,12 +32,24 @@ namespace Utility
 
             scalar angle, rot_angle;
             Vector3 rot_axis;
+            Vector3 ex = {1,0,0};
+            Vector3 ey = {0,1,0};
 
+            bool antiparallel = false;
             for( int i = 0; i < c->images[0]->nos; ++i )
             {
                 rot_angle = Engine::Vectormath::angle(spins_1[i], spins_2[i]);
                 rot_axis = spins_1[i].cross(spins_2[i]).normalized();
 
+                if(std::abs(rot_angle - Constants::Pi) < 1e-4) // If spins are antiparallel, we choose an arbitrary rotation axis
+                {
+                    antiparallel = true;
+                    if(std::abs(spins_1[i].dot(ex)) - 1 > 1e-4)
+                        rot_axis = ex;
+                    else
+                        rot_axis = ey;
+                }
+
                 // If they are not strictly parallel we can rotate
                 if( rot_angle > 1e-8 )
                 {
@@ -55,6 +68,8 @@ namespace Utility
                     }
                 }
             }
+            if(antiparallel)
+                Log(Log_Level::Warning, Log_Sender::All, "For the interpolation of antiparallel spins an arbitrary rotation axis has been chosen.");
         }
     }//end namespace Configuration_Chain
 }//end namespace Utility