Helium is the second most abundant gas in the universe, besides hydrogen, in compounds such as Lithium hydride. Helium is also a noble gas which is known to be chemically inert and indeed the least reactive element. For that reason, it is used for numerous applications, such as cooler for nuclear power plants, inert atmosphere to realize chemical reactions, for cryogenic purposes or for Helium-tagged Infrared Photo Dissociation (IRPD). In that latter, Helium atoms are attached to ions in different quantities and released through laser energy absorption. What is expected in this technique is that forming a layer of Helium does not affect what is inside. This work focuses on this specific question: although its inertness, can the number and shape of He around an ion or molecule change its properties?
In order to provide an answer, we studied through Density Functional Theory (DFT) and post-Hartree-Fock methods the differences in energy, frequency, charge, and spin of 15 He clusters at different distances, free or encapsulating Li or Li+. Our results suggest that, first, the presence of Lithium, both as charged species or radical, favors the formation of He clusters with a preference for specific shapes. Second, different shapes can influence what is inside differentially in all aspects, where a charged ion is more influenced than a radical inside a Helium cluster, i.e., the charge distribution is more influenced than the spin distribution.
Although Helium is chemically inert, our data suggest that a Helium environment can slightly influence a molecule or a charge in a shape-dependent manner. These results have relevance in at least three different fields: 
-When performing reactions in Helium atmosphere or helium droplets, the molecules' properties could be affected, hence the reactivity. 
-As the vibration frequencies are drastically changing with the presence of Lithium, we could probably deduce in space from Helium hydride spectra the Helium environment around it. 
-In He-tagged IRPD, the charge and spin of the ion could be influenced by the number of He and, to a lesser extent, by the shape.

Shape effect of He cluster on charge and spin: Li and Li+ as radical and ion model

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