10.1021/jp903912q.s001 Andrew Kerridge Andrew Kerridge Nikolas Kaltsoyannis Nikolas Kaltsoyannis Are the Ground States of the Later Actinocenes Multiconfigurational? All-Electron Spin−Orbit Coupled CASPT2 Calculations on An(η<sup>8</sup>-C<sub>8</sub>H<sub>8</sub>)<sub>2</sub> (An = Th, U, Pu, Cm) American Chemical Society 2009 equilibrium geometry actinide element sandwich molecules plutonocene ground states CASPT 2 wave functions Ground States MJ energy spectrum multiconfigurational character thorocene 5 f series Actinocenes Multiconfigurational ground state UCOT Spin CASPT 2 Calculations uranocene 2009-07-30 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Are_the_Ground_States_of_the_Later_Actinocenes_Multiconfigurational_All_Electron_Spin_Orbit_Coupled_CASPT2_Calculations_on_An_sup_8_sup_C_sub_8_sub_H_sub_8_sub_sub_2_sub_An_Th_U_Pu_Cm_/2840251 Spin−orbit free and spin−orbit coupled CASPT2 wave functions and energies are presented for the ground and low-lying excited states of four actinide element sandwich molecules; thorocene (ThCOT<sub>2</sub>), uranocene (UCOT<sub>2</sub>), plutonocene (PuCOT<sub>2</sub>), and curocene (CmCOT<sub>2</sub>). Spin−orbit coupling is found to make little difference to the equilibrium geometry of uranocene and plutonocene but has a significant effect on the energy spectrum of all the systems considered here other than thorocene. In all cases, however, the spin−orbit free ground states make the dominant contribution to their spin−orbit coupled counterparts. Following work presented in <i>J. Phys. Chem. A</i> <b>2009</b>, <i>113</i>, 2896, the variation in the multiconfigurational character of the ground-state wave functions as the 5f series is crossed is quantified via the occupation of the a<sub>u</sub>/b<sub>1u</sub> (e<sub>2u</sub>) metal-ring bonding and antibonding natural orbitals. The ground state of plutonocene is found to be nondegenerate with |<i>M</i><sub><i>J</i></sub>| = 0, in agreement with its temperature-independent paramagnetism.