Multireference ab initio methods and generalized gradient approximation functional predict that the ground state of Ru-2 is (5)Sigma(+)(g) state. However, hybrid functionals cannot predict the ground state correctly due to unreasonable overstabilization of high spin states. To calculate the relative energy order of the electronic states of Ru-2, it is essential to consider the dynamic correlation effect using a basis set of reasonable size. Only multireference configurational interaction method can reasonably calculate the bond dissociation energy of Ru-2. The ground state of Ru-2 has a quadruple bond. (C) 2013 Elsevier B.V. All rights reserved.