Density Functional and Multiconfigurational Ab Initio Study of the Ground and Excited States of Os2

Multiconfigurational ab initio methods predict that the (5)Pi(u) state as the ground state instead of the (7)Delta(u) state. Although multiconfigurational perturbation theory correctly predicts the ground state, they overestimate the bond dissociation energy (BDE). Only multireference configuration interaction method can reasonably calculate the BDE. The spin-orbit effect on the spectroscopic constants is not significant. The results calculated by density functional theory (DFT) vary significantly depending on the selection of a DFT functional. No DFT functional gives the same energy ordering as calculated by the second-order multiconfigurational perturbation theory (CASPT2). The old generalized gradient approximations functionals are well suited for predicting the ground state and calculating the bond length and the vibrational frequency of Os-2. According to the CASPT2 calculation, the ground state of Os-2 has a quadruple bond. (C) 2014 Wiley Periodicals, Inc.