Exploring the Superhalogen Properties of Polynuclear Structures without Halogen Ligands: A Combined and DFT Study on Triple-Bridged [MgL] (L = -OCN, -SCN) Anions.

Via combined and DFT calculations, 16 [Mg(OCN)] and [Mg(SCN)] isomers are studied to explore their potentials as superhalogens. The results of high-level OVGF calculations verify that these systems are superhalogen anions due to their high VDE values which lie within 6.74-7.04 eV and 5.73-6.27 eV for [Mg(OCN)] and [Mg(SCN)], respectively. The results of low-cost methods, either or DFT, are generally consistent with those of OVGF, and some of them demonstrate promising accuracy. The best performance of low-cost methods comes from (HF+MP2)/2, which represents the average value of the MP2 and HF results. In the aspect of DFT, five functionals (CAM-B3LYP, ωB97X-D3, M06, M06-2X, and B2PLYP) provide the most accurate results when compared to OVGF. Thus, these low-cost methods could be used to calculate the VDE value of the future systems of larger size. Useful information about the VDEs of [Mg(OCN)] and [Mg(SCN)] could be obtained from the analysis of the extra electron density and HOMO as well as spin density.