The primary use of **sym**
is to facilitate symmetrical placement of copies of a structure
within related volume data,
usually a density map. The symmetry of the data can be
specified in either of two ways:

- with BIOMT (PDB REMARK 350) records in the PDB file of the molecule model (example)
- by using the command
**volume**with its symmetry option. This information is retained in files saved in Chimera map format.

- generate the symmetry-related copies of
*molmodel* - initiate dynamic updating of the
*molmodel*copies when the original molecule is moved. Generally this is done while*refmodel*(the volume model) is deactivated and thus frozen in place. All copies will be moved using the symmetry matrices applied in the coordinate frame of*refmodel*.

The keyword **contact** indicates that only copies with any atom
within *contact-dist* of the original molecule should be generated.
The keyword **range** indicates that only copies with centers within
*range-dist* of the center of the original molecule should be generated.
A structure's center is defined as the center of its bounding box.
It may be necessary to use one of these options or simply to specify
fewer matrices to avoid creating too many atoms.

A secondary use of **sym** is to generate BIOMT-described
copies of a molecule model, where that model is specified as both
*molmodel* and *refmodel*.
Since a model cannot be moved relative to itself, the
dynamic updating does not come into play in this situation.

The command **~sym** removes the copies of *molmodel*
generated with **sym**.
If the volume *refmodel* is closed, the copies of *molmodel*
will no longer update to preserve symmetry when the original molecule is
moved.

BIOMT matrices can be added to PDB files with a text editor. The image shows twelve copies of myosin arranged helically, as specified by the following twelve matrices for PDB entry 1i84 (the first is simply an identity matrix that does not specify an additional copy):

REMARK 350 BIOMOLECULE: 1 REMARK 350 APPLY THE FOLLOWING TO CHAINS: S, T, U, V, W, Z REMARK 350 BIOMT1 1 1 0 0 0 REMARK 350 BIOMT2 1 0 1 0 0 REMARK 350 BIOMT3 1 0 0 1 0 REMARK 350 BIOMT1 2 0 -1 0 0 REMARK 350 BIOMT2 2 1 0 0 0 REMARK 350 BIOMT3 2 0 0 1 0 REMARK 350 BIOMT1 3 -1 0 0 0 REMARK 350 BIOMT2 3 0 -1 0 0 REMARK 350 BIOMT3 3 0 0 1 0 REMARK 350 BIOMT1 4 0 1 0 0 REMARK 350 BIOMT2 4 -1 0 0 0 REMARK 350 BIOMT3 4 0 0 1 0 REMARK 350 BIOMT1 5 0.866025 -0.5 0 0 REMARK 350 BIOMT2 5 0.5 0.866025 0 0 REMARK 350 BIOMT3 5 0 0 1 145 REMARK 350 BIOMT1 6 -0.5 -0.866025 0 0 REMARK 350 BIOMT2 6 0.866025 -0.5 0 0 REMARK 350 BIOMT3 6 0 0 1 145 REMARK 350 BIOMT1 7 -0.866025 0.5 0 0 REMARK 350 BIOMT2 7 -0.5 -0.866025 0 0 REMARK 350 BIOMT3 7 0 0 1 145 REMARK 350 BIOMT1 8 0.5 0.866025 0 0 REMARK 350 BIOMT2 8 -0.866025 0.5 0 0 REMARK 350 BIOMT3 8 0 0 1 145 REMARK 350 BIOMT1 9 0.866025 0.5 0 0 REMARK 350 BIOMT2 9 -0.5 0.866025 0 0 REMARK 350 BIOMT3 9 0 0 1 -145 REMARK 350 BIOMT1 10 -0.5 0.866025 0 0 REMARK 350 BIOMT2 10 -0.866025 -0.5 0 0 REMARK 350 BIOMT3 10 0 0 1 -145 REMARK 350 BIOMT1 11 -0.866025 -0.5 0 0 REMARK 350 BIOMT2 11 0.5 -0.866025 0 0 REMARK 350 BIOMT3 11 0 0 1 -145 REMARK 350 BIOMT1 12 0.5 -0.866025 0 0 REMARK 350 BIOMT2 12 0.866025 0.5 0 0 REMARK 350 BIOMT3 12 0 0 1 -145There is a Quicktime movie of the myosin copies moving symmetrically at the Chimera Web site.

See also:
**matrixcopy**,
**volume**,
**Volume Viewer**,
**Multiscale Models**,
**Unit Cell**