“…-Vertical integration: the method consists in propagating the boundary conditions into the corona from the bottom boundary, layer by layer Cuperman, Ofman, and Semel, 1989, 1990a, 1990bWu et al, 1990;Démoulin, Cuperman, and Semel, 1992;Song et al, 2006Song et al, , 2007. -MHD evolutionary techniques: based on the low plasma-β MHD equations, an initial configuration including electric currents is relaxed to a nonlinear force-free state owing to resistivity, also know as stress-and-relax models (Yang, Sturrock, and Antiochos, 1986;Mikic, Barnes, and Schnack, 1988;Schnack et al, 1990;McClymont and Mikic, 1994;Roumeliotis, 1996;Valori, Kliem, and Keppens, 2005;Valori, Kliem, and Fuhrmann, 2007;Valori et al, 2010;Jiang et al, 2011). -Optimization: the basic principle is to minimise a functional containing the force-free constraint as well as the solenoidal constraint to relax an initial configuration towards a nonlinear force-free state (Wheatland, Sturrock, and Roumeliotis, 2000;Wiegelmann, 2004;Wiegelmann et al, 2005Wiegelmann et al, , 2008Wiegelmann, Inhester, and Sakurai, 2006;Inhester and Wiegelmann, 2006;Wiegelmann and Neukirch, 2006;Tadesse, Wiegelmann, and Inhe 2009;Mysh'yakov and Rudenko, 2009;Wiegelmann and Inhester, 2010;Fuhrmann et al, 2011).…”