POSCAR file

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 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 1 1 Direct 0.00 0.00 0.00 0.25 0.25 0.25The 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. (If 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 supplies the number of atoms per atomic species (one number for each atomic species).

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). As in the file KPOINTS
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

*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 (see section 6.29).

Entering velocities by hand is rarely done, except for the case IBRION=0 and SMASS=-2 (see section 6.30). In this case the initial velocities are kept constant allowing to calculate the energy for a set of different linear dependent positions (for instance frozen phonons, section 9.9, dimers with varying bond-length, section 9.6). 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 (/fs, no multiplication with the scaling factor in this case) or (direct lattice vector/timestep).

*Mind:* For IBRION=0 and SMASS=-2
the actual steps taken are
POTIM*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(, 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.