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. 2018 Feb 19;9:722. doi: 10.1038/s41467-018-03200-4

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — Crystal structures of HeN₄. Energetically favorable structures of HeN₄ with space groups C2/c (a), P $\bar{1}$ (b) and I4₁/a (c). Small blue and large black spheres represent N and He atoms, respectively. Three-dimensional valence electron localization functions with isosurface value of 0.95 for the C2/c structure (d), and of 0.9 for the P $\bar{1}$ (e) and the I4₁/a (f) structures