The VASP POSCAR format is a bit "odd" compared to most other structure formats in that it doesn't contain a required specification of the chemical elements - only the coordinates. The elements are implicitly read from the pseudopotentials in the POTCAR file when the VASP calculation is run.
If you have a VASP POSCAR file that you would like to import into VNL, but for whatever reason you do not have a corresponding POTCAR file, then you can create a dummy POTCAR file with just the lines that ATK looks for to determine the element.
So let's say you have a structure of NaCl, which in POSCAR format would look like
NaCl
1.0
0.00000000 2.82028000 2.82028000
2.82028000 0.00000000 2.82028000
2.82028000 2.82028000 0.00000000
1 1
Direct
0.00000000 0.00000000 0.00000000 # Na
0.50000000 0.50000000 0.50000000 # Cl
(This is the slightly edited file produced by the
VASP Scripter in VNL).
Then you can make a file called POTCAR in the same directory, with the contents
TITEL x x Na
TITEL x x Cl
The order of the elements must of course match the order of the species in the POSCAR file.
Note: In a real POTCAR file, the lines are actually "TITEL = PAW Au"; you can of course also use this format, the only important things is the keyword TITEL and then ATK looks for the 4rd element of items separated by spaces to be the element symbol.
