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Subsections
For fixed-cell optimization, specify calculation='relax' and
add namelist &IONS. All options for a single SCF calculation apply,
plus a few others. You
may follow a structural optimization with a non-SCF band-structure
calculation (since v.4.1, you do not need any longer to update the
atomic positions in the input file for non scf calculation).
See Example 03.
Specify calculation='md', the time step dt, and possibly the number of MD stops nstep.
Use variable ion_dynamics in namelist &IONS for a fine-grained control
of the kind of dynamics. Other options for setting the initial
temperature and for thermalization using velocity rescaling are
available. Remember: this is MD on the electronic ground state, not
Car-Parrinello MD.
See Example 04.
"A common mistake many new users make is to set the time step dt
improperly to the same order of magnitude as for CP algorithm, or
not setting dt at all. This will produce a ``not evolving dynamics''.
Good values for the original RMW (RM Wentzcovitch) dynamics are
dt
= 50 ÷ 70
. The choice of the cell mass is a delicate matter. An
off-optimal mass will make convergence slower. Too small masses, as
well as too long time steps, can make the algorithm unstable. A good
cell mass will make the oscillation times for internal degrees of
freedom comparable to cell degrees of freedom in non-damped
Variable-Cell MD. Test calculations are advisable before extensive
calculation. I have tested the damping algorithm that I have developed
and it has worked well so far. It allows for a much longer time step
(dt=
100 ÷ 150
) than the RMW one and is much more stable with very
small cell masses, which is useful when the cell shape, not the
internal degrees of freedom, is far out of equilibrium. It also
converges in a smaller number of steps than RMW." (Info from Cesar Da
Silva: the new damping algorithm is the default since v. 3.1).
See also examples/VCSexample.
Next: 5.3 Nudged Elastic Band
Up: 5 Using PWscf
Previous: 5.1 Electronic structure calculations
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Paolo Giannozzi
2010-05-07