Obtaining Intrinsically Occupied Free-Space Superatom States in an Encapsulated Ca(2)N Nanotube

[Image: see text] By considering a group of atoms as a whole, the superatom state concept has been proposed to understand complex chemical systems. Superatom states distributed in free space are important in determining the interactions between superatoms and also the reactions of a superatom system with other external molecules. Unfortunately, all free-space superatom states reported to date are unoccupied states, which strongly limit their applications. In this study, we predict that both occupied and unoccupied free-space superatom states exist in an encapsulated Ca(2)N nanotube. In this composite system, the inner Ca(2)N nanotube provides anionic electrons in free space inside the tube, which form occupied s-, p-, and d-like superatom states. The outer carbon nanotube layer provides an effective protection for these free-space superatom states from the ambient environment. Such protected superatom states with flexible occupation statuses are expected to have a great potential in various application fields including catalysis and electronics.