Stability issues in computational screening of carbon nanostructures: illustrations on La endohedrals

Three important stability problems frequently met in calculations of carbon nanostructures are outlined. The three relative-stability issues concern: isomeric carbon clusters, non-isomeric metallofullerenes differing in the number of the encapsulated metal atom yet placed in still isomeric carbon cages, and monometallofullerenes with different metals encapsulated in one selected carbon cage. The related computational approaches are illustrated with endohedrals based on La and its congeners located in (Formula presented.), (Formula presented.) and (Formula presented.) cages, both IPR (isolated-pentagon rule) and non-IPR ones, evaluated using various DFT treatments. It is also concluded that among the La congeners, Ac can produce the second best encapsulation energy, just after La.