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.