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\bibitem{Callewaert_etal2017} {\textquotedblleft}\textit{Surface states and positron annihilation spectroscopy: results and prospects from a first-principles approach}{\textquotedblright}. Callewaert V, Saniz R, Barbiellini B, Partoens B, Journal of physics : conference series \textbf{791}, 012036 (2017).   http://doi.org/10.1088/1742-6596/791/1/012036

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