Do we have to consider the translational symmetry when constructing a POSCAR for VASP? For example, I have a conventional FCC unit cell containing 14 atoms (8 on corners and 6 on the face). However, some of the atoms are in the same position due to the symmetry, such as the (0,0,0) and (1,1,1) atom. Do we have to only show the atoms with unique positions?
FCC contains only 4 atom, not 14. 8 atoms in the corners are shared by 8 adjacent unit cells. Thus, one unit cell has 1/8th or each of these 8 atoms. And the atoms on the faces are shared by two adjacent unit cells. Therefore, atom share for one unit cell will be 1/2 for the 6 face atoms. Thus, in total there would be 4 atoms per unit cell.
So, define only the four unique atoms: (0,0,0), (0.5,0.5,0), (0.5,0,0.5), and (0,0.5,0.5).
Uttam Pal is right, but I want to answer this question in a more direct sense:
Yes, VASP always uses periodic boundary conditions and you have to delete duplicate atoms to avoid crashes. Note that you can also use only 1 atom at (0,0,0) for an fcc crystal by choosing the appropriate minimal unit cell with vectors
0.5, 0.0, 0.5
0.5, 0.5, 0.0
0.0, 0.5, 0.5
Yes, and VASP will tell you in no uncertain terms that something is wrong.
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