Description of a known asymmetry bug

Former-commit-id: 51d44df76e5294e860795320ede73f7953427ae6

BUGS.rst

@@ -0,0 +1,31 @@
+Known bugs
+===========
+
+Scattering result asymmetry
+---------------------------
+The lattice scattering code (such as finitesqlatzsym-scattery.py) produces
+asymmetric results where one should not get them, due to the system symmetry.
+
+It seems that the asymmetry appears mostly in the y-direction (i.e.
+for example the scattering/absorption cross section at k = (kx, ky, kz)
+is not exactly the same as k = (kx, -ky, kz).
+
+What has been checked (hopefully):
+ - The flip operators for electric waves
+ - Some weird kind of rounding or other numerical error depending on
+   the position ordering of the matrix (randomized indices give
+   the same asymmetry).
+
+What has not been checked (so well):
+ - The x,y,z-flip operators for magnetic waves (i.e. are they really 
+   supposet to bejust the
+   same as the operators for electric waves, just with opposite sign?) 
+ - zplane_pq_y
+ - the translation operators
+
+
+Singular value asymmetry
+------------------------
+Similar as the scattering result asymmetry, although not necessarily 
+only in the y-direction?
+