You shoud first run siesta with C6H6.fdf as input to find the relaxed geometry
(there will be errors in the frequencies if you start from a non-relaxed
geometry).

We will assume that you have done this first step, and that the relaxed coordinates
are in relax-system.fdf. Since this system is a molecule, we do not need to
run fcbuild to generate a supercell. When run with file ForceConstants.fdf as input,
Siesta will generate a .FC file with the force constants. (Note that ForceConstants.fdf
"%includes" relax-system.fdf.)  This run will take quite a long time on a normal computer,
so we have provided the file C6H6.FC.prebuilt for you.

Finally, we need to run "vibra" with vibra.fdf as input to compute the vibration
frequencies. Note that only the Gamma point is specified in the "bands" section.
If you plot the "dispersion" you will see only a column of points. Note the huge
"gap". Where does it come from?


(If you use the pre-built FC file, remember to make a copy called C6H6.FC before running
vibra.)


