# SYMMETRY

Molecular (*not* crystallographic) symmetry. The author can specify
a point group or a set of symmetry operations (this could be useful for
a helical molecule or one in a non-standard orientation.)

The content is the symmetry operators as (4*3) matrices (ARRAY). These
should have the form [**R**|**t**] where **R**
premultiplies the coordinates
and **t** is a column translation vector. It is up to the author whether
they give a complete set of operators (e.g. 48 for Oh) or whether they
give just the group generators. The identity matrix can be assumed to be
present in all cases.

SYMMETRY supports CONVENTION/DICTNAME and can therefore distinguish between
Schoenflies and H-M conventions.
The following BUILTINs for XVAR are provided:

**POINTGROUP** Pointgroup. There are two conventions
(H-M and Schoenflies); the latter is normally used by everyone except
crystallographers. I suggest that you format them like D2d or Oh.
**AXIS** Principal axis of the molecule: X, Y or Z.

### Content Model

- Optional XHTML description.
- Optional/repeatable ARRAY/XLIST/ARRAY. The XLIST could be used to
contain a list of symmetry operators, each of which is a matrix described in
an ARRAY. Alternatively ARRAY could contain a list of text-based operators
in crystallograpic format (e.g. "X,-Y,1/2+Z").

## Content

**array** -- A very flexible matrix/array/geometry container.
**xhtml** -- A hypertext container for use in TecML and CML.
**xlist** -- A very flexible generic list/tree/table container.
**xvar** -- A generic, flexible, container for scalar information.

**ATTRIBUTES**

**CONTENT DECLARATION**

**Tag Minimization**-
Open Tag:
*REQUIRED*

Close Tag: *REQUIRED*

## Parent Elements

**mol** -- Toplevel container for molecular information.

**Top Elements**

**All Elements**

**Tree**

cml DTD