mirror of https://gitlab.com/QEF/q-e.git
47 lines
2.5 KiB
Plaintext
47 lines
2.5 KiB
Plaintext
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This example illustrates how to use pw.x to compute the equilibrium
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geometry of a simple molecule, CO, and of an Al (001) slab.
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The calculation proceeds as follows (for the meaning of the cited input
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variables see the appropriate INPUT_* file)
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1) make a geometry relaxation for CO molecule performing a series of
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self-consistent calculations and computing the forces on atoms
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(input=co.rx.in, output=co.rx.out).
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The molecule is put in a cubic box of side 12 Bohr.
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Note that ibrav=0 therefore the Bravais lattice fundamental vectors
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are read after cards 'CELL_PARAMETERS' (where we also specify the
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type of symmetry, cubic or hexagonal). The cell parameter is not
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specified in celldm(1), but deduced from Bravais lattice vectors.
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Calculation is set to 'relax', so specifying that a structural
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relaxation is performed.
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While approaching the minimum, the scf threshold (initially
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conv_thr=1.0d-8) will automatically become smaller (stricter
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convergence) because of the need to evaluate correctly forces and
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the tiny energy differences involved in the relaxation. This
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tightening of the scf threshold is however limited by the upscale=10
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statement that specifies that conv_thr ccannot become smaller than
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its starting value / upscale**2 (=1.0d-10 in the present example).
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2) make a geometry relaxation for a Al (001) slab performing a series of
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self-consistent calculations and computing the forces on atoms
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(input=al001.rx.in, output=al001.rx.out).
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This is a 7-atomic-layer slab separated by about 4 vacuum layers.
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The unit cell in tetragonal (ibrav=6) with celldm(1)=alat_fcc/sqrt(2).
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Calculation is set to 'relax'.
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While approaching the minimum, the scf threshold (initially
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conv_thr=1.0d-6) will automatically become smaller (stricter
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convergence) because of the need to evaluate correctly forces and
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the tiny energy differences involved in the relaxation. This
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tightening of the scf threshold is however limited by the upscale=10
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statement that specifies that conv_thr ccannot become smaller than
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its starting value / upscale**2 (=1.0d-8 in the present example).
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3) make a geometry relaxation for the same Al (001) slab used in step 2
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performing a series of self-consistent calculations, computing the
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forces on atoms and evolving the atomic positions according to Newton
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equation. Whenever a velocity component is opposite to the corresponding
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force component, the velocity is stopped.
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(input=al001.mm.in, output=al001.mm.out).
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