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# POSCAR

This file contains the lattice geometry and the ionic positions, optionally also starting velocities and predictor-corrector coordinates for a MD-run. The usual format is:

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
Selective dynamics
Cartesian
0.00 0.00 0.00 T T F
0.25 0.25 0.25 F F F
Cartesian
0.01 0.01 0.01
0.00 0.00 0.00
optionally predictor-corrector coordinates
given on file CONTCAR of MD-run
....
....


or

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


The first line is treated as a comment line (you should write down the name of the system). The second line provides a universal scaling factor (lattice constant), which is used to scale all lattice vectors and all atomic coordinates (of this value is negative it is interpreted as the total volume of the cell). On the following three lines the three lattice vectors defining the unit cell of the system are given (first line corresponding to the first lattice vector, second to the second, and third to the third). The sixth line specifies the constituting elements (in the order how they appear in the POTCAR file). The seventh line supplies the number of atoms per atomic species (one number for each atomic species). The ordering must be consistent with the POTCAR and the INCAR file. The eighth line switches to selective dynamics (only the first character is relevant and must be S or s). This mode allows to provide extra flags for each atom signaling whether the respective coordinate(s) of this atom will be allowed to change during the ionic relaxation. This setting is useful if only certain shells around a defect or layers near a surface should relax.

Mind: The selective dynamics input tag is optional: the seventh line supplies the switch between cartesian and direct lattice if the Selective dynamics tag is omitted.

The seventh line (or eighth line if selective dynamics is switched on) specifies whether the atomic positions are provided in cartesian coordinates or in direct coordinates (respectively fractional coordinates). Only the first character on the line is significant and the only key characters recognized by VASP are C, c, K or k for switching to the cartesian mode. The next lines give the three coordinates for each atom. In the direct mode the positions are given by
${\displaystyle {{\vec R}}=x_{1}{{\vec a}}_{1}+x_{2}{{\vec a}}_{2}+x_{3}{{\vec a}}_{3}}$
where ${\displaystyle {{\vec a}}_{{1...3}}}$ are the three basis vectors, and ${\displaystyle x_{{1...3}}}$ are the supplied values. In the cartesian mode the positions are only scaled by the factor ${\displaystyle s}$ on the second line of the POSCAR file ${\displaystyle {{\vec R}}=s\left({\begin{array}{c}x_{1}\\x_{2}\\x_{3}\end{array}}\right).}$

The ordering of these lines must be correct and consistent with the number of atoms per species on the sixth line. If your are not sure whether you have a correct input please check the OUTCAR file, which contains both the final components of the vector ${\displaystyle {{\vec R}}}$ and the positions in direct (fractional) coordinates. If selective dynamics are switched on each coordinate a triplet is followed by three additional logical flags determining whether to allow changes of the coordinates or not (in our example the 1. coordinate of atom 1 and all coordinates of atom 2 are fixed). If the line selective dynamics is removed from the POSCAR file this flag will be ignored (and internally set to .T.).

Mind: The flags refer to the positions of the ions in direct coordinates, no matter whether the positions are entered in cartesian or direct coordinates. Therefore, in the example given above the first ion is allowed to move into the direction of the first and second direct lattice vector.

If no initial velocities are provided, the file may end here. For molecular dynamics the velocities are initialised randomly according to a Maxwell-Boltzmann distribution at the initial temperature TEBEG.

Entering velocities by hand is rarely done (one important case is e.g. using the tags IBRION=0 and SMASS=-2), but if done the velocities are provided at the end of the POSCAR file very similalry to the positions. As previously the first line supplies a switch between cartesian coordinates and direct coordinates. On the next lines the initial velocities are provided. They are given in units ${\displaystyle \AA }$/fs (no multiplication with the scaling factor in this case) or (direct lattice vector/timestep). When the initial velocities are supplied in the POSCAR and the tags IBRION=0 and SMASS=-2) are set, the velocities are kept constant during the MD allowing to calculate the energy for a set of different linear dependent positions (for instance frozen phonons and dimers with varying bond-length).

Mind: For IBRION=0 and SMASS=-2 the actual steps taken are POTIM times read velocities. To avoid ambiguities, set POTIM to 1. In this case the velocities are simply interpreted as vectors, along which the ions are moved. For the cartesian switch, the vector is given in cartesian coordinates (${\displaystyle \AA }$, no multiplication with the scaling factor in this case) for the direct switch the vector is given in direct coordinates.

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.