The crystal structure is specified by the lattice vectors and the atomic coordinates. The lattice vectors are given as rows of coordinates with respect to a cartesian basis. The unit lenght is an atomic length unit, i.e. the Bohr radius.
In addition to the lattice vectors, the volume may be specified. If you provide a volume, the lattice vectors will be rescaled such that they span a unit cell with the given volume. If you leave out the volume statement, the lattice vectors are assumed to be given in atomic units.
The lattice vectors and optionally the volume must be enclosed within a begin latticevecs and end latticevecs statement.
Example:
begin latticevecs coord 7.22362499411784 -5.04013632834720 0.00000000000000 coord 0.00000000000000 3.95068558900000 2.76187238600000 coord 0.00000000000000 -3.95068558900000 2.76187238600000 volume 157.3848 end latticevecs
The coordinates of the atoms must be enclosed by the statements begin coordinates and end coordinates. Each new element is indicated with a keyword newtype. All coord statements following a newtype statement refer to the new element, until the next newtype keyword is encountered.
The newtype keyword is followed by the atomic type, e.g. O,Si,C, by the spin polarization of the initial charge configuration for this atom, and the size of the muffin-tin sphere (used only for projected DOS, and, if unspecified, is set to 1.0 a.u by default). For instance
newtype Si 0.5 2.4
will define a Si atom. For setting up the initial charge density, a definition of spin polarization of will be used, with a default of 0.2. The muffin-tin sphere around the silicon atom will have a radius of 2.4 a.u. For non-spin-polarized calculations, set the spin polarization to 0.
By default, the coordinates following coord are understood to be relative to the basis vectors. To facilitate the input of molecular crystals, it is possible to give them in absolute, cartesian coordinates (a.u.) by specifying the keyword coordinates_absolute OUTSIDE the begin/end of the coordinates.
NOTE: This is a change from pre-5.0 versions of paratec! Previously one specified coordinates absolute, whereas now an underscore is required (i.e. coordinates_absolute)!
Example:
begin coordinates newtype O 0.17 coord 0.15693515545994 0.10044534718500 -0.10044534718500 coord -0.15693515545994 -0.10044534718500 0.10044534718500 newtype Si 0.0 coord 0.5 0.5 0.5 end coordinates