The magnetic coupling in the manganese dimer has been investigated with multiconfigurational computational schemes. Whereas the perturbational approaches present severe deviations to the expected Heisenberg behaviour, variational techniques based on difference dedicated configuration interaction (DDCI) perfectly fit a Heisenberg spectrum corresponding to antiferromagnetic coupling. This shows that the study of weak magnetic coupling as in Mn-2 is at (or beyond) the limit of applicability of perturbational techniques. Potential energy curves are obtained for the six low-lying R states by adding the DDCI energy difference to the reference potential energy surface calculated with coupled cluster techniques for the monoconfigurational 11 Sigma(+)(g) g state. (c) 2008 Elsevier B.V. All rights reserved.