VCA
VCA = [real array]
Default: VCA = read from the POTCAR file
Description: VCA is short for the virtual crystal approximation; the tag allows to "weight" each species found in the POTCAR file.
The tag VCA has to be supplied for each atom type or species found in the POTCAR respectively {{TAG|POSCAR} file. It weights the corresponding POTCAR files according to the values given in the INCAR file, with the default obviously being 1. For instance the formal valency found in the POTCAR files is multiplied by the supplied values, and likewise the augmentation charges, and the non-local pseudopotential strenght paramters are scaled by the supplies values.
It is possible to use this flag to perform calculations in the framework of the virtual crystal approximation. Say you want to simulate Sn doping in a Ge lattice. Using a POTCAR file with a Ge and Sn data set and the following POSCAR file, this can be achieved:
cd:
1.00000000000000
2.82173 2.82173 0.00000
0.00000 2.82173 2.82173
2.82173 0.00000 2.82173
Ge Sn
2 2
Direct
0.00 0.00 0.00
0.25 0.25 0.25
0.00 0.00 0.00
0.25 0.25 0.25
If LVCA is set to
VCA = 0.99 0.01
the Ge atoms are weighted with a weight of 0.99, whereas the Sn atoms are weighted by 0.01 (see [1]). The implementation in VASP closely follows the methotology suggested by Bellaiche and Vanderbilt.[2]
Caveats: Unfortunately results of such VCA calculations are often not very reliable.