1
2This example illustrates how to use pw.x to compute the equilibrium
3geometry of a simple molecule, CO, and of an Al (001) slab.
4
5The calculation proceeds as follows (for the meaning of the cited input
6variables see the appropriate INPUT_* file)
7
81) make a geometry relaxation for CO molecule performing a series of
9   self-consistent calculations and computing the forces on atoms
10   (input=co.rx.in, output=co.rx.out).
11   The molecule is put in a cubic box of side 12 Bohr.
12   Note that ibrav=0 therefore the Bravais lattice fundamental vectors
13   are read after cards 'CELL_PARAMETERS' (where we also specify the
14   type of symmetry, cubic or hexagonal). The cell parameter is not
15   specified in celldm(1), but deduced from Bravais lattice vectors.
16   Calculation is set to 'relax', so specifying that a structural
17   relaxation is performed.
18   While approaching the minimum, the scf threshold (initially
19   conv_thr=1.0d-8)  will automatically become smaller (stricter
20   convergence) because of the need to evaluate correctly forces and
21   the tiny energy differences involved in the relaxation. This
22   tightening of the scf threshold is however limited by the upscale=10
23   statement that specifies that conv_thr ccannot become smaller than
24   its starting value / upscale**2 (=1.0d-10 in the present example).
25
262) make a geometry relaxation for a Al (001) slab performing a series of
27   self-consistent calculations and computing the forces on atoms
28   (input=al001.rx.in, output=al001.rx.out).
29   This is a 7-atomic-layer slab separated by about 4 vacuum layers.
30   The unit cell in tetragonal (ibrav=6) with celldm(1)=alat_fcc/sqrt(2).
31   Calculation is set to 'relax'.
32   While approaching the minimum, the scf threshold (initially
33   conv_thr=1.0d-6)  will automatically become smaller (stricter
34   convergence) because of the need to evaluate correctly forces and
35   the tiny energy differences involved in the relaxation. This
36   tightening of the scf threshold is however limited by the upscale=10
37   statement that specifies that conv_thr ccannot become smaller than
38   its starting value / upscale**2 (=1.0d-8 in the present example).
39
403) make a geometry relaxation for the same Al (001) slab used in step 2
41   performing a series of self-consistent calculations, computing the
42   forces on atoms and evolving the atomic positions according to Newton
43   equation. Whenever a velocity component is opposite to the corresponding
44   force component, the velocity is stopped.
45   (input=al001.mm.in, output=al001.mm.out).
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47