[Wien] Problems with velocity calculations

Dear all,
the purpose of my calculations is to get dE/dk of charge carriers for specific
band number in point (x,y,z).
For this after common wien calculations I use two methods (I do calculations
for Bi2Te3 in symmetric point (x,x,x)):

1. I create klist file with points with coordinates:
(x+dx,y,z)
(x-dx,y,z)
(x,y+dx,z)
(x,y-dx,z)
(x,y,z+dx)
(x,y,z-dx)
Do lapw1 (and lapwso), and get E(...)'s
Calculate dE/dk: dE/dk_x = (E(x+dx,y,z)-E(x-dx,y,z))/2dx and e.t.c.

2. I clear klist file and put there point with coordinate (x,y,z)
Do optic (with pmat flag)

I have few problems:
1. In first method results dramatically depend on dx, for dx=1e-6 and dx=1e-7
results differ by two times... no convergence.
2. I get completely different results by this two methods.
I understand that I could lose some factor, but in first method I have
symmetric vector, like (x,x,x) as expected, in second I have not symmetric
result, I get vector like (x,y,z).

I have few questions:
1. What method is right (optimal)?
2. Why results are not equalt?
3. Why I haven't convergence in first method?
4. Why in second method I receive non symmetric vector? (Even if result is in
cartesian coordinates I expect vector like (0,0,x) for this point in Bi2Te3).

Hope somebody can help me or give advice.
_______________________________________________
Wien mailing list
Wien@zeus.theochem.tuwien.ac.at
http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
SEARCH the MAILING-LIST at:
http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html