[Wien] External Electric Static Potential

[Chiung-Yuan Lin]

Dear wien2k users,
  I have done a surface (slab) calculation and would like to add a
(external) localized electric static potential near the surface (in the
vacuum region). All I want is to do a one-time diagonalization under
the potential V_{slab} + V_{external}. Does anyone have experience
doing this?
  The way I am thinking is to modify the interstitial potential contained
in the my case.vns and then run lapw1 under such a potential. The
interstitial
potential is written in terms of partial waves like the following example:

       0    0    0  -0.514528955284E-01 0.000000000000E+00
       0    0   -1  -0.199581895321E+00 0.136295067798E-16
       0    0   -2  0.966346059588E-01 0.936790650016E-17
      -1    0    0   0.847906315430E-02-0.887491207822E-02
      -1    1    0    0.847906315430E-02 0.887491207822E-02
      -1    0   -1   0.223869299564E-02-0.655993625842E-02
      -1    1   -1   0.223869299564E-02 0.655993625842E-02
      -1    0   -2  -0.443569349002E-02 0.633324702410E-04

I guess the first 3 integers in each row represent the K vector of
UG's eqn. (2.10), and the last two real numbers are related to V_{K}.
My questions are:

Does the r vector of UG's eqn. (2.10) use the same coordinate system
(including the origin) as the atom positions in the case.struct file?
How about the K vector?

What is the unit of K in terms of these integers?

Do the two real numbers represent the Re(V_{K}) and Im(V_{K})
or are defined in another way?

I appreciate very much if you can either answer my questions or even
find an easier way to add such a potential.

Thank you
Chiung-Yuan