A colloquium on the variational method applied to excitons in 2D materials Maurício F.C. Martins Quintela and Nuno M.R. Peres Eur. Phys. J. B, 93 12 (2020) 222 Published online: 02 December 2020 DOI: 10.1140/epjb/e2020-10490-9