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Choosing the Plane Wave Cutoff
After getting familiar with the input and output of CPMD, we now have
to run a test on the validity of the pseudopotential and its cutoff
requirements for the subsequent calculations (geometry optimization
and molecular dynamics). For that purpose we make a copy of the
input file from the wavefunction optimization and add the keyword
PRINT FORCES ON to the &CPMD section and
then re-run the wavefunction optimization for a series of wavefunction
cutoff values ranging from 10ryd up to 200-500ryd (depending on the
available memory. Take note of the values of total energy, and force
for each cutoff value. Since the convergence behavior can be different
for each pseudopotential (and each property), we exchange the pseudopotential
file as well using H_BHS_LDA.psp as replacement. When plotting
the relative errors from those calculations, we get a graph similar to
the following:
|
Energy and forces have a the same convergence trend,
but for the forces we have a not as regular behavior
in the relative error. Next we see that for both
pseudopotentials, the relative error in the force
reaches some kind of plateau starting at 50ryd, with
the Martins-Troullier (MT) potential behaving a bit better
than the BHS-type pseudopotential parameters. For
problems requiring even more accurate forces, a choice
of 100-120ryd would be better. Please note, however,
that these errors only document the convergence behavior
of the pseudopotential with respect to the basis set
size (= cutoff), they do not include systematic
error contributions, e.g. the error from using
DFT altogether in or from the choice of pseudization
cutoff radius (which can be chose to make a ``softer''
pseudopotential at the expense of the transferability
of the pseudopotential). Typical errors on bond lengths
or lattice constants from DFT calculations relative
to experiment are of the order of one to a few percent.
For all subsequent calculations on hydrogen we will use
the H_MT_LDA.psp file and a wavefunction
cutoff of 60ryd.
Subsections
Next: Relation between Plane Wave
Up: The Basics: Running CPMD,
Previous: Other Output Files
Contents
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Costas Bekas
2008-09-04