more on defect mig

diff --git a/posic/thesis/defects.tex b/posic/thesis/defects.tex
index a7c8a24..b0073b6 100644 (file)
--- a/posic/thesis/defects.tex
+++ b/posic/thesis/defects.tex
@@ -533,27 +533,31 @@ Three different migration paths are accounted in this work, which are shown in f
 The first migration investigated is a transition of a \hkl<0 0 -1> into a \hkl<0 0 1> dumbbell interstitial configuration.
 During this migration the carbon atom is changing its silicon dumbbell partner.
 The new partner is the one located at $\frac{a}{4}\hkl<1 1 -1>$ relative to the initial one.
-Two of the three bonds to the next neighboured silicon atoms are preserved while the breaking of the other bond and the formation of a new bond is observed.
+Two of the three bonds to the next neighboured silicon atoms are preserved while the breaking of the third bond and the accompanying formation of a new bond is observed.
 The carbon atom resides in the \hkl(1 1 0) plane.
-This transition involves the bond-centerd configuration.
-Due to symmetry it is enough to consider the transition from the bond-centered to the \hkl<1 0 0> configuration.
+This transition involves an intermediate bond-centerd configuration.
+Due to symmetry it is enough to consider the transition from the bond-centered to the \hkl<1 0 0> configuration or vice versa.
 In the second path, the carbon atom is changing its silicon partner atom as in path one.
-In addition, the dumbbell configuration is changing
-
-As a last migration path, the \hkl<0 0 -1>
-
-Since the starting and final structure, which are both local minima of the potential energy surface are known, the aim is to find the minimum energy path from one local minimum to the other one.
-One method is to move the diffusing atom stepwise from the starting to the final position and only allow relaxation in the plane perpendicular to the direction of the vector connecting its starting and final position.
-No constraints are applied to the remaining atoms.
-
-Different approaches were used to compute migration paths and energies.
-CRT erklaeren
-Nur C atom constrainted
-Problem, 1) relaxation of host matrix 2) abrubt changes in energy and configuration
-as can be seen in ...
-Thus, 'all atom CRT' but! relaxation perpendicular to delta step ...
-
-Results and comparison with diffusion experiments.
+However, the the trajectory of the carbon atom is no longer proceeding in the \hkl(1 1 0) plane.
+The orientation of the new dumbbell configuration is transformed from \hkl<0 0 -1> to \hkl<0 -1 0>.
+Again one bond is broken while another one is formed.
+As a last migration path, the defect is only changing its orientation.
+The silicon dumbbell partner remains.
+The bond to the face-centered silicon atom at the bottom of the unit cell breaks and a new one is formed to the face-centered atom at the forefront of the unit cell.
+
+Since the starting and final structure, which are both local minima of the potential energy surface, are known, the aim is to find the minimum energy path from one local minimum to the other one.
+One method to find a minimum energy path is to move the diffusing atom stepwise from the starting to the final position and only allow relaxation in the plane perpendicular to the direction of the vector connecting its starting and final position.
+No constraints are applied to the remaining atoms in order to allow relaxation of the surrounding lattice.
+To prevent the remaining lattice to migrate according to the displacement of the defect an atom far away from the defect region is fixed in all three coordinate directions.
+However, it turned out, that this method tremendously failed applying it to the present migration pathways and structures.
+Abrupt changes in structure and free energy occured among relaxed structures of two successive displacement steps.
+For some structures even the expected final configurations were never obtained.
+Thus, the method mentioned above was adjusted adding further constraints in order to obtain smooth transitions, either in energy as well as structure is concerned.
+In this new method all atoms are stepwise displaced towards their final positions.
+Relaxation of each individual atom is only allowed in the plane perpendicular to the last individual displacement vector.
+The modifications used to add this feature to the VASP code and a short instruction on how to use it can be found in appendix \ref{app:patch_vasp}.
+
+In figure ... results of the \hkl<0 0 -1> to \hkl<0 0 1> migration are presented.
 
 \section{Combination of point defects}