Another important aspect is including static correlation effects to treat the degeneracy in the MF 6 molecules. Most previous studies of MF 6 were based on single-reference methods such as the density functional theory (DFT) or coupled cluster theory. These methods are sufficient for closed-shell systems but may encounter problems when there is significant multi-reference effect In the context of the vibrational coupling with the JT effect, multi-reference methods with the SOC were employed by Balasubramanian's group for metal clusters , by Mondal for the first-row transition-metal trihalides 43, 44, and by Koseki's group for third-row transition-metal tetrahydrides . However, such an approach has not been applied for MF 6 molecules.