## SYMMETRY: Symmetry Group Input


The SYMMETRY directive is used (optionally) within the compound GEOMETRY
directive to specify the point group for the molecular geometry or space
group for the crystal structure. The general form of the directive, as
described above within the general form of the GEOMETRY directive, is as
follows:
```
   [SYMMETRY [group] <string group_name>|<integer group number> \
   [setting <integer setting>] [print] \  
          [tol <real tol default 1d-2>]]
```
The keyword group is optional, and can be omitted without affecting how
the input for this directive is processed. However, if the SYMMETRY
directive is used, a group name must be specified by supplying an entry
for the string variable `<group_name>` or `<group number>`. The latter is
useful for the space groups discussed in the section below. The group
name should be specified as the standard Schöflies symbol. Examples of
expected input for the variable group\_name include such entries as:

-   c2v - for molecular symmetry *C*<sub>2*v*</sub>
-   d2h - for molecular symmetry *D*<sub>2*h*</sub>
-   Td - for molecular symmetry *T*<sub>*d*</sub>
-   d6h - for molecular symmetry *D*<sub>6*h*</sub>

The SYMMETRY directive is optional. The default is no symmetry (i.e.,
*C*<sub>1</sub> point group). Automatic detection of point group
symmetry is available through the use of autosym in the GEOMETRY
directive main line (discussed in [Keywords for the GEOMETRY directive](Keywords-for-the-GEOMETRY-directive.md)).
Note: if the SYMMETRY directive is present the autosym keyword is
ignored.

If only symmetry-unique atoms are specified, the others will be
generated through the action of the point group operators, but the user
if free to specify all atoms. The user must know the symmetry of the
molecule being modeled, and be able to specify the coordinates of the
atoms in a suitable orientation relative to the rotation axes and planes
of symmetry. The section [Geometry Examples](Geometry-examples.md) lists a number of examples of the `GEOMETRY`
directive input for specific molecules having symmetry patterns
recognized by NWChem. The exact point group symmetry will be forced upon
the molecule, and atoms within 10<sup>−3</sup> A.U. of a symmetry
element (e.g., a mirror plane or rotation axis) will be forced onto that
element. Thus, it is not necessary to specify to a high precision those
coordinates that are determined solely by symmetry.

The keyword print gives information concerning the point group
generation, including the group generators, a character table, the
mapping of centers, and the group operations.

The keyword tol relates to the accuracy with which the symmetry-unique
atoms should be specified. When the atoms are generated, those that are
within the tolerance, tol, are considered the same.