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The POTCAR file essentially contains the pseudopotential for each atomic species used in the calculation. If the number of species is larger than one, one simply concatenates the POTCAR files of the species. On a UNIX machine you might type the line
cat ~/pot/Al/POTCAR ~/pot/C/POTCAR ~/pot/H/POTCAR > POTCAR
Mind: It is a frequent error to give the wrong ordering in the POTCAR file!
For VASP version ≥ 3.2, the POTCAR file also contains information about the atoms, i.e., their mass POMASS, their number of valence electrons ZVAL, the energy of the reference configuration for which the pseudopotential was created etc. If tags for the mass and valence exist in the INCAR file, they are checked against the parameters found in the POTCAR file and error messages are printed. The POTCAR files also contain a default energy cutoff (ENMAX and ENMIN); therefore, it is no longer necessary to specify ENCUT in the INCAR file. The value in the INCAR file overwrites the default in the POTCAR file. For POTCAR files with more than one species, the maximum cutoffs (ENMAX or ENMIN) are used for the calculation.