Keywords: user_guide

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# 3 Input data

Input data for `cp.x` is organized into 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, electron dynamics
&IONS : | ionic variables, ionic dynamics
&CELL (optional): | variable-cell dynamics

The `&CELL` namelist may be omitted for fixed-cell calculations. 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:

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

).

Explanations for the meaning of variables `ibrav` and `celldm`, as well as on alternative ways to input structural data, are contained in files `Doc/INPUT_CP.*`. These files are the reference for input data and describe a large number of other variables as well. 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
> 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.

Subsections

- 3.1 Data files
- 3.2 Format of arrays containing charge density, potential, etc.

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