Another use of meta-atoms is in the recognition of linear atoms within symmetry definitions. Some external symmetry definitions depend on whether there are linear atoms within the structure. For example, if a carbon has a the same group with all four ligands, it can have a external symmetry of 4. But, if those four ligands are linear structures, then the external symmetry increases by a factor of 3 to 12.
In the recognition of symmetry, the linear structures are first identified within the species. Then if, in examining the structure associated with the symmetry, the desired ligands are identified as linear groups (a LinearAtom).
Within the database, the basic linear structures are defined. However, in the recognition of a complete linear structure, a linear structure can be a line of these basic linear structures. For example, a triple bonded carbon is a basic linear structure. However, two triple bonded carbons linked together is also a linear structure. These two basic linear structures are then combined to make a 'meta' linear structure.
Single atoms are, by definition, linear structures.
Triple bonded carbons: RCCR (L-CtC)
Nitrile: RCN (L-CN)
All single atoms