Variational optimization of the 2DM: approaching three-index accuracy using extended cluster constraints Brecht Verstichel, Ward Poelmans, Stijn De Baerdemacker, Sebastian Wouters and Dimitri Van Neck Eur. Phys. J. B, 87 3 (2014) 59 Published online: 10 March 2014 DOI: 10.1140/epjb/e2014-40788-x