Run first fcbuild with si54.fdf as input. That will generate the file FC.fdf, which
is "%included" by si54-siesta.fdf. When Siesta is run with si54-siesta.fdf as 
input, it will produce (after some time) a file called si54.FC, which contains
the force constants.

Finally, run "vibra" with si54.fdf as input to compute the phonon frequencies.
To plot the phonon dispersion relation, run "gnubands" with si54.bands as input,
and use gnuplot to plot the resulting data file.
