Ab initio study of the (2 × 2) phase of barium on graphene Cesare Tresca, Nikolay I. Verbitskiy, Alexander Grüneis and Gianni Profeta Eur. Phys. J. B, 91 7 (2018) 165 Published online: 16 July 2018 DOI: 10.1140/epjb/e2018-90141-6