Keywords: user_guide

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## 3.1 Input data

Input data is organized as several namelists, followed by other fields (``cards'') introduced by keywords. The namelists are

&CONTROL: | general variables controlling the run
&SYSTEM: | structural information on the system under investigation
&ELECTRONS: | electronic variables: self-consistency, smearing
&IONS (optional): | ionic variables: relaxation, dynamics
&CELL (optional): | variable-cell optimization or dynamics

Optional namelist may be omitted if the calculation to be performed does not require them. This depends on the value of variable `calculation` in namelist &CONTROL. Most variables in namelists have default values. Only the following variables in &SYSTEM must always be specified:

`nat` | (integer) | number of atoms in the unit cell
`ntyp` | (integer) | number of types of atoms in the unit cell
`ecutwfc` | (real) | kinetic energy cutoff (Ry) for wavefunctions.

plus the variables needed to describe the crystal structure, e.g.:

`ibrav` | (integer) | Bravais-lattice index
`celldm` | (real, dimension 6) | crystallographic constants

Alternative ways to input structural data are described in files `PW/Doc/INPUT_PW.*`. For metallic systems, you have to specify how metallicity is treated in variable `occupations`. If you choose `occupations='smearing'`, you have to specify the smearing type `smearing` and the smearing width `degauss`. Spin-polarized systems are as a rule treated as metallic system, unless the total magnetization, `tot_magnetization` is set to a fixed value, or if occupation numbers are fixed (`occupations='from input'` and card OCCUPATIONS).

Detailed explanations of the meaning of all variables are found in files `PW/Doc/INPUT_PW.*`. Almost all variables have default values, which may or may not fit your needs.

Comment lines in namelists can be introduced by a "!", exactly as in fortran code.

After the namelists, you have several fields (``cards'') introduced by keywords with self-explanatory names:

> ATOMIC_SPECIES
> ATOMIC_POSITIONS
> K_POINTS
> CELL_PARAMETERS (optional)
> OCCUPATIONS (optional)

The keywords may be followed on the same line by an option. Unknown fields are ignored. See the files mentioned above for details on the available ``cards''.

Comments lines in ``cards'' can be introduced by either a ``!'' or a ``#'' character in the first position of a line.

Note about k-points: The k-point grid can be either automatically generated or manually provided as a list of k-points and a weight in the Irreducible Brillouin Zone only of the Bravais lattice of the crystal. The code will generate (unless instructed not to do so: see variable `nosym`) all required k-points and weights if the symmetry of the system is lower than the symmetry of the Bravais lattice. The automatic generation of k-points follows the convention of Monkhorst and Pack.

Next: 3.2 Data files Up: 3 Using PWscf Previous: 3 Using PWscf Contents