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Subsections

8.3 Input data

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!

8.3.0.1 ''Where can I find the crystal structure/atomic positions of XYZ?''

'''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".

8.3.0.2 ''Where can I find the Brillouin Zone/high-symmetry points/irreps for XYZ?''

'''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.

8.3.0.3 ''Where can I find Monkhorst-Pack grids of k-points?''

'''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.

8.3.0.4 ''How do I perform a calculation with spin-orbit interactions?''

'''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)

8.3.0.5 ''How can I choose parameters for variable-cell molecular dynamics?''

'''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 up previous contents
Next: 8.4 Parallel execution Up: 8 Frequently Asked Questions Previous: 8.2 Pseudopotentials Contents
Paolo Giannozzi 2010-04-08