OptionalatomIndex of the atom in the structure which corresponds to the environment.
If we are considering structures properties, this is undefined.
The global environment index.
Index of the structure which this Indexes.environment corresponds to.
Indexes related to a single entry in a property.
This can exists in either structure mode (in which case
environnement === structureand atom is undefined); or atom mode. In atom mode, the environnement is the index of the entry in the property, and structure/atom define to which atom in which structure the entry correspond.