Structures of RgFn (Rg = Xe, Rn, and Element 118. n = 2, 4.) Calculated by Two-component Spin−Orbit Methods. A Spin−Orbit Induced Isomer of (118)F4

Ab initio calculations of RgF n (Rg = Xe, Rn, and Element 118; n = 2, 4) were performed using relativistic effective core potentials and two-component HF, MP2, CCSD, and CCSD(T) methods. Geometries were optimized at the HF level with and without effective spin−orbit operators. The D 4 h structures of all tetrafluorides and the linear difluorides are local minima with and without spin−orbit interactions. Spin−orbit contributions makes the T d form of (118)F 4 another local minimum with the energy comparable to that of the D 4 h one. The spin−orbit interactions stabilize the (118) fluorides by a significant margin (∼2.0 eV) and the Rn fluorides by 40−60% (0.2∼ 0.4 eV) of the stabilization energy obtained at the corresponding scalar relativistic level. For (118)F 4, the vibrational frequency of the B 2u mode of the D 4 h form decreases from 143 to 20 cm -1 upon inclusion of the spin−orbit interactions, and a doubly degenerate mode of the T d structure, which is stable only with the inclusion of spin−orbit interactions, has the frequency of 34 cm -1, indicating that the (118)F 4 molecule is very flexible due to spin−orbit effects.