Europium dimer: van der Waals molecule with extremely weak antiferromagnetic spin coupling

High-level ab initio calculations reveal that the Eu-2 dimer is a van der Waals molecule with extremely weak antiferromagnetic spin coupling. The Heisenberg spin-exchange model, validated by the multireference configuration interaction method, is used to construct the full set of model interaction potentials for the states with the total spin S ranging from 0 to 7 at the coupled cluster level of theory. This model establishes the singlet (1)Sigma(+)(g) state as the ground one of the dimer with the binding energy of 710 cm(-1), the vibrational frequency of 23 cm(-1) and the effective spin-coupling constant J estimated approximately -0.3 cm(-1).