The POSCAR file is a mandatory VASP input file. It is a plain text file and contains at least the lattice geometry and the ionic positions. Optionally, also starting velocities for a molecular-dynamics simulation can be provided here. This file shares its format with VASP output file CONTCAR. That may contain an additional section with predictor-corrector coordinates necessary for restarting molecular-dynamics runs.
Creating a POSCAR file is often the starting point of VASP-supported research. It can be written manually or obtained from various online materials and crystallographic databases providing a download in the POSCAR file format.
In its simplest form the POSCAR file contains basic information about the lattice, per-species number of ions and their positions. This is sufficient in most situation where a VASP calculation is started from scratch. Have a look at this example for cubic boron nitride:
Cubic BN 3.57 0.0 0.5 0.5 0.5 0.0 0.5 0.5 0.5 0.0 B N 1 1 Direct 0.00 0.00 0.00 0.25 0.25 0.25
As indicated by the text coloring there are four blocks corresponding to the following file contents:
The first line is reserved for a free user comment, e.g. a system description.
Scaling factor and lattice
In this block the first line specifies a universal lattice scaling factor . The next three lines define the lattice vectors. Each line holds the unscaled Cartesian components of one lattice vector. The actual lattice vectors and (in ) are the product of the given numbers with the lattice scaling factor. Set the universal scaling factor to 1 if you want to enter the lattice vectors directly and avoid any additional scaling.
Ion species and numbers:
This section defines how many ions of each species are present. The first line lists the species names, the second specifies the number of ions for each species. The given order should match the order of species appearing in the POTCAR file.
|Warning: If machine-learned force fields are used (ML_LMLFF=.TRUE.), it is not possible to give the same name to different groups of atoms in the POSCAR file.|
Finally, the ion positions (in ) are listed in this section. The first line selects one of the two possible modes how the coordinates and given in the following lines are interpreted:
- "Direct" means the positions are provided in direct (fractional) coordinates:
where is the position vector of an ion.
- "Cartesian" specifies that positions are provided in a Cartesian coordinate system. However, the actual ion positions are also multiplied with the universal scaling factor, i.e.
The total number of lines with positions must match the total number of ions given in the previous section. The ion species are also derived from there, i.e. in the example above it is implied that the list of positions contains one boron ion, followed by one nitrogen nuclei.
Full format specification
The POSCAR file format is constructed from multiple sections arranged in a predefined order. Some sections contain only a single line, others span over many lines, some may even be omitted. The following list defines the section order and their contents:
|Section||Number of lines
||mandatory||The first line is reserved for a free user comment, e.g. a system description. The maximum line length is 40 characters, extra characters are truncated.|
||mandatory||This line may contain one or three numbers. If one number is provided it specifies a universal lattice scaling factor . It is multiplied with the three vectors in the following section to obtain the lattice vectors of the unit cell. Also, the ion positions are scaled with this factor if the "Cartesian" mode is selected (see section "Ion positions"). If the number is negative, it is interpreted as the desired cell volume. Then, the scaling factor is computed automatically to obtain the desired volume. If three numbers are provided in this line they act as individual scaling factors for the x-,y- and z-Cartesian components for the lattice vectors (and "Cartesian" mode ion positions). In this case all three numbers must be positive.|
|Lattice||3||mandatory||This sections contains three lines defining the lattice vectors. Each line holds the unscaled Cartesian components of one lattice vector. The actual lattice vectors and (in ) are the product of the given numbers with the lattice scaling factor |
|Species names||1||optional||This line lists the species of the present ions. The given order should match the order of species appearing in the POTCAR file. This line is optional, if omitted the species names are taken from the POTCAR file.
|Ions per species||1||mandatory||This mandatory line lists how many ions of each species are present. The given order should match the order of species appearing in the POTCAR file.|
|Selective dynamics||1||optional||If the line after the "Ions per species" section contains |
||1 + #atoms||mandatory||Here, the ion positions (in ) are listed. The first line selects one of the two possible modes how the coordinates and given in the following lines are interpreted:
Actually, only the first character on the line is significant and the only key characters recognized are
The total number of lines with positions must match the total number of ions given in the "Ions per species" section. The ion species are also derived from there, e.g. if the "Ions per species" section lists
If the selective dynamics feature is enabled on each coordinate triplet is followed by three additional logical flags, i.e. each is either
... Selective dynamics Cartesian 0.00 0.00 0.00 T F T 1.27 0.98 0.32 F T F ...
Here, the first atom is allowed to move into the direction of the first and third direct lattice vector. The second atom may only move in the second lattice vector direction.
If no initial velocities are provided, the file may end here.
||8||from CONTCAR only1||Contains the lattice vectors and and their velocities. Lattice velocities occur when in molecular dynamics simulations lattice vectors are treated as dynamic variables, i.e., are allowed to change over time (IBRION=0 together with ISIF=3). When written to the CONTCAR file the section starts with a line containing the string |
|Ion velocities||1 + #atoms||optional||Here initial velocities for all ions can be provided. The input format is similar to the section "Ion positions" above. The first line determines the input mode which is either "Direct" or "Cartesian". In contrast to the reading of ion positions there is no multiplication with the scaling factor applied in the "Cartesian" mode. Another minor difference is the interpretation of the contents of the first line in this section. In addition to |
|MD extra||variable||from CONTCAR only1||The predictor-corrector coordinates are only provided to continue a molecular dynamic run from a CONTCAR file of a previous run, they can not be entered by hand.|
1 "from CONTCAR only": This section is usually not entered manually by the user. It appears in the CONTCAR file output at the end of VASP runs which involve ionic steps and is intended for restarting a previous calculation.
Precision and symmetry
VASP determines the symmetry of the system from the POSCAR file. It is a common mistake to enter the positions with insufficient precision (too few digits). To make the best use of the symmetry routines in VASP, it is strongly recommended to specify the positions (and lattice parameters) in the POSCAR file with at least 7 significant digits (but preferably more). Internal tests for symmetry operations are done against a user-supplied value for the precision, specified by SYMPREC (defaults to 10-5). Hence, 5 significant digits are absolutely borderline and can cause serious issues in the automatic symmetry determination, for instance, finding some but not all generators for the symmetry group. Also, "noise" in the positions might grow during relaxations, so that sometimes, upon reading the CONTCAR file, some symmetry operations are not found. All these issues are best avoided by making the initial POSCAR file as accurate as possible.
If you have a POSCAR file with the positions written with low precision and would like to reconstruct with higher precision, we recommend using a symmetry package, such as spglib, to find the symmetries given a certain precision, symmetrizing the lattice vectors and positions and writing the POSCAR file with a higher number of significant digits. This can be done using pymatgen (which interfaces with spglib) to symmetrize the structure and write it to a POSCAR file, see example on github.
Cubic BN 3.57 0.00000000 0.50000000 0.50000000 0.50000000 0.00000000 0.50000000 0.50000000 0.50000000 0.00000000 B N 1 1 Selective dynamics Cartesian 0.00000000 0.00000000 0.00000000 T T F 0.25000000 0.25000000 0.25000000 F F F Cartesian 0.01000000 0.01000000 0.01000000 0.00000000 0.00000000 0.00000000 optionally predictor-corrector coordinates given on file CONTCAR of MD-run .... ....
fcc Si 3.9 0.50000000 0.50000000 0.00000000 0.00000000 0.50000000 0.50000000 0.50000000 0.00000000 0.50000000 1 cartesian 0.00000000 0.00000000 0.00000000
MgO Fm-3m (No. 225) 1.0 2.606553 0.000000 1.504894 0.868851 2.457482 1.504894 0.000000 0.000000 3.009789 Mg O 1 1 direct 0.000000 0.000000 0.000000 Mg 0.500000 0.500000 0.500000 O