[Wien] Supercell Coulomb potential using single unitcell results

[zcl@physik.hu-berlin.de]

Dear all,
I want to calculate the mean inner potential (MIP) of crystal, such as
TiC. The MIP is defined as average Coulomb potential of the crytal while
the zero point is selected in the vacuum at infinite distance from the
crytal. I want to use  two diffrent methods, the first one is  I set up a
supercell include several small unitcell and a vacuum layer at the top.
Then run the cycles to get the electron density. After change in case.in0 
 NR2V to R2V and run lapw0 again. I will get the Coulomb potential in file
case.vcoul.The potential in vacuum will be set up as zero point to
caculate the mean inner potential of the crystal . However , this will
cost a lot of time as we need to calculate the supercell for the whole
cycles.
I think another method is that only one unit cell will be set up and run
the cycles to get the electron density of the unit cell. Then I want to
use this electron density to set up a large supercell and add a vacuum
layer at the top. Only one apw0 will run to output the Coulomb potential
of this supercell, including the cyrstal and the vacuum. But I don't know
how to use the electron density calculated from one unit cell to form a
large supercell and calculate the potential by lapw0.
Many thanks!

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Changlin Zheng
HU-Berlin, Germany