[Pw_forum] Help in using xcrysden for making structures

[Gabriele Sclauzero]

Dear Neel,

Neel Singh wrote:
> hi,
> I need some help in drawing crystal using xcrysden. Sir/Madam, let us 
> take the case of FCC for which ibrav=2, and celldm(1)=9.000. Now if i 
> know that i have 18 atoms in my unit cell and i know the Cartesian 
> co-ordinates{x,y,z} of each atom in angstrom/bhor. I want to know that 
> how can i draw my unit cell with giving positions of all the atoms 
> without using the symmetries by xcrysden??. Means like if i specifiy,

I think that Xcrysden does not use rotational symmetries to visualize atoms (Tone, correct 
me if I'm wrong). What it can do is to repeat periodically the atomic basis (i.e. the 
minimal set of atoms which, repeat periodically with translations by lattice vectors, give 
you the full infinite crystal) to show a nicely cut unit cell, which sometimes includes 
translationally equivalent atoms.
In the ATOMIC_POSITIONS list you should include ONLY non-equivalent atoms.

> & system
> ibrav=2, celldm(1)=9.00, nat=1,......
> ATOMIC POSITIONS{angstrom}
> A  0  4.5 0
> 
> now if we visualise this type of structure with Xcrysden, we found a 
> number of atoms genrated due to symmetry. If i make a file like
>  & system
> ibrav=2, celldm(1)=9.00, nat=18,......
> ATOMIC POSITIONS{angstrom}
> A  0  4.5 0
> /
> 18 times we have specified the co-ordinates
> /
> /
> A  4.5 0 0
> Xcrysden will give warning like atom 10 and 90 are very close like that.

It means that either you specified wrong positions for one or more atoms, or maybe you 
included translationally equivalent atoms in the list (i.e. at least a couple of atoms 
which have positions which differ by a lattice vector).
BTW, you said that you have 18 atoms, not 90...

> Sir i want to know is there any way so that i can make my PWSCF input 
> file with angstrom/bohr postions such that with one atomic postion 
> Xcrysden will show only one atom??

You have to specify the correct list of atomic positions. That's up to you, XCrysden 
cannot be used to build input files at the moment. When you have the correct atomic 
positions, you can switch between the 2 visualizations modes (atomic basis only, or 
"nicely cut unit cell" with some replicas of the atoms in the basis) using the menu bar on 
the bottom.

If you want more help, please post a complete input file, otherwise we cannot understand 
what mistakes you made in building it.

Regards,

GS


> Means i will give 18 atomic postions and xcrysden will produce 18 atom 
> unit cell without using symmatries.
> 
> Thanks for your help,
> Neel Singh,
> Univeristy of Delhi,
> India
> 
> 
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| Gabriele Sclauzero, PhD Student                  |
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