
This example illustrates how to use pw.x to compute the equilibrium
geometry of a simple molecule, CO, and of an Al (001) slab.

The calculation proceeds as follows (for the meaning of the cited input
variables see the appropriate INPUT_* file)

1) make a geometry relaxation for CO molecule performing a series of
   self-consistent calculations and computing the forces on atoms
   (input=co.rx.in, output=co.rx.out).
   The molecule is put in a cubic box of side 12 Bohr. 
   Note that ibrav=0 therefore the Bravais lattice fundamental vectors
   are read after cards 'CELL_PARAMETERS' (where we also specify the
   type of symmetry, cubic or hexagonal). The cell parameter is not
   specified in celldm(1), but deduced from Bravais lattice vectors.
   Calculation is set to 'relax', so specifying that a structural 
   relaxation is performed.
   While approaching the minimum, the scf threshold (initially
   conv_thr=1.0d-8)  will automatically become smaller (stricter
   convergence) because of the need to evaluate correctly forces and
   the tiny energy differences involved in the relaxation. This
   tightening of the scf threshold is however limited by the upscale=10
   statement that specifies that conv_thr ccannot become smaller than
   its starting value / upscale**2 (=1.0d-10 in the present example).

2) make a geometry relaxation for a Al (001) slab performing a series of
   self-consistent calculations and computing the forces on atoms
   (input=al001.rx.in, output=al001.rx.out).
   This is a 7-atomic-layer slab separated by about 4 vacuum layers.
   The unit cell in tetragonal (ibrav=6) with celldm(1)=alat_fcc/sqrt(2).
   Calculation is set to 'relax'.
   While approaching the minimum, the scf threshold (initially
   conv_thr=1.0d-6)  will automatically become smaller (stricter
   convergence) because of the need to evaluate correctly forces and
   the tiny energy differences involved in the relaxation. This
   tightening of the scf threshold is however limited by the upscale=10
   statement that specifies that conv_thr ccannot become smaller than
   its starting value / upscale**2 (=1.0d-8 in the present example).

3) make a geometry relaxation for the same Al (001) slab used in step 2
   performing a series of self-consistent calculations, computing the 
   forces on atoms and evolving the atomic positions according to Newton 
   equation. Whenever a velocity component is opposite to the corresponding
   force component, the velocity is stopped. 
   (input=al001.mm.in, output=al001.mm.out).

