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<H2><A ID="SECTION00041000000000000000">
3.1 Input data</A>
</H2>
<P>
Input data
is organized as several namelists, followed by other fields (``cards'')
introduced by keywords. The namelists are
<P>
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT">&CONTROL:</TD>
<TD ALIGN="LEFT">general variables controlling the run</TD>
</TR>
<TR><TD ALIGN="LEFT">&SYSTEM:</TD>
<TD ALIGN="LEFT">structural information on the system under investigation</TD>
</TR>
<TR><TD ALIGN="LEFT">&ELECTRONS:</TD>
<TD ALIGN="LEFT">electronic variables: self-consistency, smearing</TD>
</TR>
<TR><TD ALIGN="LEFT">&IONS (optional):</TD>
<TD ALIGN="LEFT">ionic variables: relaxation, dynamics</TD>
</TR>
<TR><TD ALIGN="LEFT">&CELL (optional):</TD>
<TD ALIGN="LEFT">variable-cell optimization or dynamics</TD>
</TR>
</TABLE>
<BR>
Optional namelist may be omitted if the calculation to be performed
does not require them. This depends on the value of variable
<TT>calculation</TT>
in namelist &CONTROL. Most variables in namelists have default values. Only
the following variables in &SYSTEM must always be specified:
<P>
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT"><TT>nat</TT></TD>
<TD ALIGN="LEFT">(integer)</TD>
<TD ALIGN="LEFT">number of atoms in the unit cell</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT>ntyp</TT></TD>
<TD ALIGN="LEFT">(integer)</TD>
<TD ALIGN="LEFT">number of types of atoms in the unit cell</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT>ecutwfc</TT></TD>
<TD ALIGN="LEFT">(real)</TD>
<TD ALIGN="LEFT">kinetic energy cutoff (Ry) for wavefunctions.</TD>
</TR>
</TABLE>
<BR>
plus the variables needed to describe the crystal structure, e.g.:
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT"><TT>ibrav</TT></TD>
<TD ALIGN="LEFT">(integer)</TD>
<TD ALIGN="LEFT">Bravais-lattice index</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT>celldm</TT></TD>
<TD ALIGN="LEFT">(real, dimension 6)</TD>
<TD ALIGN="LEFT">crystallographic constants</TD>
</TR>
</TABLE>
<BR>
Alternative ways to input structural data are described in files
<TT>PW/Doc/INPUT_PW.*</TT>. For metallic systems, you have to specify
how metallicity is treated in variable <TT>occupations</TT>.
If you choose <TT>occupations='smearing'</TT>, you have
to specify the smearing type <TT>smearing</TT> and the smearing width
<TT>degauss</TT>. Spin-polarized systems are as a rule treated as metallic
system, unless the total magnetization, <TT>tot_magnetization</TT>
is set to a fixed value, or if occupation numbers are fixed
(<TT>occupations='from input'</TT> and card OCCUPATIONS).
<P>
Detailed explanations of the meaning of all variables are found in files
<TT>PW/Doc/INPUT_PW.*</TT>. Almost all variables have default
values, which may or may not fit your needs.
<P>
Comment lines in namelists can be introduced by a "!", exactly as in fortran
code.
<P>
After the namelists, you have several fields (``cards'')
introduced by keywords with self-explanatory names:
<BLOCKQUOTE>
ATOMIC_SPECIES
<BR>
ATOMIC_POSITIONS
<BR>
K_POINTS
<BR>
CELL_PARAMETERS (optional)
<BR>
OCCUPATIONS (optional)
<BR>
</BLOCKQUOTE>
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''.
<P>
Comments lines in ``cards'' can be introduced by either a ``!'' or a ``#''
character in the first position of a line.
<P>
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 <TT>nosym</TT>) 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.
<P>
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