# Group Modify

- Type
- Scope
- Tolerance
- View

Change here the reference object data for a single group only.

To modify a group, click over its graphic image, or write its identification (name and number) on the Group entry. To modify a list of groups, press the button List (after creating the list of groups with Group->Select). Parameters for empty entries or Local choices remain unchanged.

To change a group name write the new name in the Group entry, followed by the group number (GAMGI needs the number to identify the group). To change the name for a list of groups, press List first and then write the new common name in the Name entry.

### Reference

GAMGI can handle groups that are just object containers (selecting Container), and coordination polyhedra, requiring a central atom (selecting Polytope).

For a single group, Reference can be Container, Polytope or Local. For a list of groups , Reference must be Local.

When Container or Polytope is selected, the group is recreated from scratch (as if using Group->Create) and its old content is removed. As this requires local information for each group, it cannot be done for a lists of groups. When Local is selected, previous features and contents are preserved, unless when specifically changed.

### Container

The primary purpose of a group container is to contain other objects. When a group container is created, it has no child objects and consequently no visual representation. The only way to select an empty group is thus to write its identification or to use the mechanisms provided by Group->Select.

To avoid this problem, when Ok is pressed and the new group is created, the dialog Group->Create is automatically replaced by Group->Link, with the new group identification already inscribed on it. How to link objects to a group is described in the section Group Link.

### Polytope

For an arbitrary cloud of atoms, defined by Central and Neighbours settings, build a coordination polyhedron defined by the smallest convex polygonal representation that surrounds all the atoms. This is often used in Crystallography to build tetrahedra, octahedra, and other small regular solids, but the method can be used to build polyhedra for an arbitrary set of atoms, independently of their position or number.

When Gamgi cannot build a 3D polyhedron, automatically tries to build a 2D atomic plane, as in Plane->Create. When a 2D plane cannot be defined, automatically tries to build a 1D atomic direction, as in Direction->Create. When a 1D direction cannot be defined, an error is shown.

### Central

Enter here the name and number of the central atom, around which the coordination polyhedron will be built. Pressing the mouse over an atom in the current layer, the atom identification is automatically written in the Central entry.

When Local is selected (the default), only one polyhedron is constructed, around the Central atom. When Global is set, Gamgi builds coordination polyhedra around all the atoms in the sampling universe with the same element as the Central atom.
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