test to illustrate the use of an external potential file.

first run the wavefunction optimization for the normal case
(single water molecule in gasphase) and create a cubefile
from the file DENSITY with cpmd2cube.

then compile the file mkextpot.f and run the program to create
the external potential. you'll need to specify the cell dimensions,
the origin and the real space mesh.

now run the wavefunction optimization for the external potential
case and create another cube file from DENSITY file.

create a difference cube file from the two density cubefiles,
e.g. using the cubman utility from the gaussian software package.

this cubefile now contains the response of the electron density
to the external potential and can be visualize with standard
tools. see e.g. http://www.theochem.ruhr-uni-bochum.de/go/cpmd-vmd.html
for some examples.

the script runme.sh performs all the commands described above,
h2o-dens-diff.vmd contains an example visualization of the 
density difference and h2o-dens-diff.png a sample snapshot image 
of that visualization.

both runs together take less than 20 minutes on an 
600MHz alpha workstation or and 1.3GHz amd athlon pc
and need about 220MB RAM.

01/2004  axel.kohlmeyer@theochem.ruhr-uni-bochum.de
tested with cpmd v3.8.4, cpmd2cube-0.1.2-cvs (+ bochum patches)
and vmd v1.8.2