Structures of RgF_n (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_4h, 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)F₄ another local minimum with the energy comparable to that of the D_4h 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)F₄, the vibrational frequency of the B_2u mode of the D_4h 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)F₄ molecule is very flexible due to spin-orbit effects.