
This example illustrates how to use pw.x to perform molecular dynamics
for an 8-atom cell of Si starting with compressed bonds along 111

The calculation proceeds as follows (for the meaning of the cited input
variables see the file 'INPUT_PW' in the directory 'pwdocs')

1) make a MD run for Si in the diamond structure in a 2-atom cell starting
   with the bond along (111) slightly compressed. Use the Gamma point only.

2) make a MD run for Si in the diamond structure in a 8-atom cell starting
   with the bond along (111) slightly compressed. Use the Gamma point only.
   Note that the two calculations do not give exactly the same results 
   because the BZ samplig is different.

3) make a MD run for Si in the diamond structure in a 2-atom cell starting
   with the bond along (111) slightly compressed. Use the Gamma and the
   3 X points for the BZ sampling. 
   It should give the same result as the calculation with 8 atoms (clearly 
   the total energy is 4 times larger)

   In all the three calculation above:
     calculation='md' specifies that a MD run is performed.
     dt=20            defines the time step in (Rydberg) atomic unit of time.
     The mass of each type of atom is specified in the cards ATOMIC_SPECIES
     (for Si, 28.086 is the atomic mass in a.m.u.)
     nstep=100        is the number of steps in the MD run.
     potential_extrapolation='wfc2' meansd that starting guess for the
     potential and the wavefunctions at the new atomic positions will 
     be extrapolated from previous history.
     nosym=.true.     states that symmetry should not be used in the MD run.

   Additional variables (such as temperature) could be set in a MD run.
   Refer to INPUT_PW for their meaning.

