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A large percentage of the problems reported to the mailing list are
caused by incorrect input data. Before reporting a problem with
strange crashes or strange results, PLEASE have
a look at your structure with XCrysDen. XCrysDen can directly
visualise the structure from both PWscf input data:
xcrysden --pwi "input-data-file"
and from PWscf output as well:
xcrysden --pwo "output-file".
Unlike most other visualizers, XCrysDen is periodicity-aware: you can
easily visualize periodically repeated cells.
You are advised to use always XCrysDen to check your input data!
'''A:''' The following site contains a lot of crystal structures:
http://cst-www.nrl.navy.mil/lattice .
"Since this seems to come up often, I'd like to point out that the
American Mineralogist Crystal Structure Database
(http://rruff.geo.arizona.edu/AMS/amcsd) is another excellent place to
find structures, though you will have to use it in conjunction with
Bilbao (http://www.cryst.ehu.es),
and have some understanding of space groups and Wyckoff positions".
'''A:''' "You might find this web site useful:
http://www.cryst.ehu.es/cryst/get_kvec.html" (info by Cyrille
Barreteau, nov. 2007). Or else: in textbooks, such as e.g. "The
mathematical theory of symmetry in solids", by Bradley and Cracknell.
'''A:''' Auxiliary code ''kpoints.x'' (in pwtools/, executable
produced by "make tools") generates uniform grids of k-points that are
equivalent to Monkhorst-Pack grids.
'''A:''' The following input variables are relevant for a spin-orbit
calculation:
noncolin=.true./.false.
lspinorb=.true./.false.
starting_magnetization (one for each type of atoms)
To make a spin-orbit calculation noncolin must be true.
If starting_magnetization is set to zero (or not given) the code
makes a spin orbit calculation without spin magnetization (it assumes
that time reversal symmetry holds and it does not calculate the
magnetization). The states are still two component spinors but the
total magnetization is zero.
If starting_magnetization is different from zero it makes a non
collinear spin polarized calculation with spin orbit. The final spin
magnetization might be zero or different from zero depending on the
system.
Furthermore to make a spin-orbit calculation you must use fully
relativistic pseudopotentials at least for the atoms in which you
think that spin orbit is large. If all the pseudopotentials are scalar
relativistic the calculation becomes equivalent to a noncolinear
calculation without spin orbit. (Andrea Dal Corso, 2007-07-27)
'''A''': "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).
Next: 8.4 Parallel execution
Up: 8 Frequently Asked Questions
Previous: 8.2 Pseudopotentials
Contents
Paolo Giannozzi
2009-10-01