How to define POSCAR file for Defects !

[mm05b028]

Hi everyone !

I wanted to learn, how can I write a POSCAR file for a defect(s) in my system

For Ex. I have a 1x1x1 supercell of a bulk UO2 system and at present my POSCAR is the following with Uranium atoms at corners and face centers & all Oxygen at tetrahedral voids.

System UO2:
5.47040000000000
1.0000000000000000 0.0000000000000000 0.0000000000000000
0.0000000000000000 1.0000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 1.0000000000000000
4 8
Selective dynamics
Direct
0.0000000000000000 0.0000000000000000 0.0000000000000000 T T T
0.5000000000000000 0.5000000000000000 0.0000000000000000 T T T
0.5000000000000000 0.0000000000000000 0.5000000000000000 T T T
0.0000000000000000 0.5000000000000000 0.5000000000000000 T T T
0.2500000000000000 0.2500000000000000 0.2500000000000000 T T T
0.2500000000000000 0.7500000000000000 0.2500000000000000 T T T
0.7500000000000000 0.2500000000000000 0.2500000000000000 T T T
0.7500000000000000 0.7500000000000000 0.2500000000000000 T T T
0.2500000000000000 0.2500000000000000 0.7500000000000000 T T T
0.2500000000000000 0.7500000000000000 0.7500000000000000 T T T
0.7500000000000000 0.2500000000000000 0.7500000000000000 T T T
0.7500000000000000 0.7500000000000000 0.7500000000000000 T T T

Now, If I want to have a Uranium vacancy only at one ( out of eight ) of the corner lattice points. Then how do I represent that in POSCAR file above.

1. Do I have to define a bigger supercell / Also how I define a POSCAR for Supercell ?
2. Also how I specify no. of atoms and their positions. ?

I will appreciate if someone can help me figuring this out or guide me how to find ans.

PS : I am new to VASP, so pardon me if my question is silly. I can;t find such thread on this topic on Forum.

Thank for your time. Hoping some response.

Thanks
Anuj

[alex]

Hello Anuj,

you have to redefine your lattice. Take a 2x2x2 supercell, you'd have 5.4704*2 in the second line and 8 times the number of atoms at the proper positions:
0 0 0 stays, 0.5 0.5 0 becomes 0.25 0.25 0 and so on. Then you choose one uranium to remove and you'll end up with 15(!) U and 32 O atoms.

Other possibility: you just go for one U atom on the faces of the crystal. For a defect this is a rather high concentration. However, good for learning the program anyway. ;-)

Cheers,

alex

[mm05b028]

Thank you Alex ,

I understand now how to play with POSCAR for Defects.

Thanks,
Anuj