My Community

I am thinking of determining the ground state structure of PZTN while N% is 20%, whose structure hasn't been reported by experiement. Thus I plan to start from different structures, and relax the lattice parameter and ions. However, I cannot find a proper ISIF to realize it, say, fix the cell shape and relax ions and volumes. Anyone has any idea? Thanks

please have in mind that VASP keeps the symmetry type of the lattice of the input POSCAR file (i.e., a cubic lattice remains cubic, a tetragonal lattice remains tetragonal,...)
for example: 'change the cell shape and the volume ' (IBRION = 3) of a tetragonal lattice will relax c/a, and V, but will never lead to an orthorhombic lattice or any other lattice type of lower symmetry (this, in some sense, fixes the cell shape).

Hence you can choose 2 ways to find the ground state geometries of your input structures:

1) do it automatically, but allow for a change in the shape
ISIF=3 #changes volume, cell shape, ionic positions
PREC= High #necessary to obtain stress tensors of the required accuracy
NSW= #number of steps
IBRION = 2 (or 1)

2) do it 'by hand' by fitting the energy parabola of a set of calculations of
cells of equal shapes, but different volumes
ISIF = 0, 1 or 2
PREC = High
NSW = ...
IBRION = 2 (or 1)

" please have in mind that VASP keeps the symmetry type of the lattice of the input POSCAR file (i.e., a cubic lattice remains cubic, a tetragonal lattice remains tetragonal,...)
for example: 'change the cell shape and the volume ' (IBRION = 3) of a tetragonal lattice will relax c/a, and V, but will never lead to an orthorhombic lattice or any other lattice type of lower symmetry (this, in some sense, fixes the cell shape). "

Unless ISYM=0, right? Also, what are the dangers in setting ISYM=0, apart from VASP using more k-points than the irreducible set of your structure, and thus taking longer?
(when only interested in the total energy).

Thanks


<span class='smallblacktext'>[ Edited Tue Apr 18 2006, 06:46PM ]</span>

yes, you are right, sorry for beeing not precise in that point. There are no dangers in setting ISYM=0, the only effect may be that the calculation takes longer because the complete BZ is taken instead of the irreducible part.