Density functional and multireference ab initio study of the ground and excited states of Ru2

Multireference ab initio methods and generalized gradient approximation functional predict that the ground state of Ru2 is 5 Σ 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 Ru2, 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 Ru2. The ground state of Ru2 has a quadruple bond.