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.