Multiconfigurational quantum chemical methods for molecular systems containing actinides

Abstract

Recent advances in computational actinide chemistry are reported in this tutorial review. Muticonfigurational quantum chemical methods have been employed to study the gas phase spectroscopy of small actinide molecules. Examples of actinide compounds studied in solution are also presented. Finally the multiple bond in the diuranium molecule and other diactinide compounds is described.

Nuclear Energy Gradients for Internally Contracted Complete Active Space Second-Order Perturbation Theory: Multistate Extensions

“…Note that taking the derivative of L PT2 with respect to λ Ω,LM yields the amplitude equation in Eq. (10). Taking the derivative of L PT2 with respect to T Ω,LM , one obtains the so-called λ equation:…”

“…[4][5][6] The CASPT2 method has been applied to a variety of chemical problems including the spectra of organic compounds, transition metal complexes, and heavy elements. 9,10 When multiple electronic states are nearly degenerate, special care has to be taken, because errors in the state-specific approach may arise as the CASPT2 states are generally a linear combination of the CASSCF reference states and may not be accurately approximated by a single CASSCF state. The multi-state (MS) CASPT2 approach has been derived from quasi-degenerate perturbation theory based on the Bloch wave operator formalism, which mixes different CASPT2 states by forming an effective Hamiltonian and diagonalizing it to obtain the mixing coefficients, 11,12 Recently the "extended" version of MS-CASPT2 (XMS-CASPT2) has been proposed 13,14 to rectify its failure near conical intersections and avoided crossings where its potential energy surfaces are sometimes singular.…”

Second-order perturbation theory with complete and restricted active space reference functions applied to oligomeric unsaturated hydrocarbons

“…When supplemented by multireference second-order perturbation theory, CASSCF/CASPT2, 16 so as to account for dynamical electron correlation effects not already included at the CASSCF level, accuracies on the order of 0.2 eV have been documented for state-energy splittings in molecules containing elements throughout the periodic table. [17][18][19][20][21][22][23][24][25] The major drawback of the CASSCF formalism is that the number of configuration state functions (CSFs) required for the multiconfigurational expansion increases factorially with the number of active electrons and active orbitals. Memory and disk storage limit the size of the active space in modern software packages to about 15 electrons in 15 orbitals, which is on the order of 1 Â 10 6 to 16 Â 10 6 CSFs depending on spin and spatial symmetry.…”

The electronic spectrum of CUONg4 (Ng = Ne, Ar, Kr, Xe): New insights in the interaction of the CUO molecule with noble gas matrices

“…For this ion (but also for non-oxide isoelectronic species such as NUN) the uranium metal center forms strong triple bonds with the axial ligands. 3,[7][8][9][10][11][12][13][14] The high stability and linear structure of these U(VI) species are well explained by the "pushing from below" model proposed by Tatsumi and Hoffmann. 15 In the linear structure the strong interaction between the semicore uranium 6p-orbitals and the oxygen 2p-orbitals brings the energy of the σ u HOMO (highest occupied molecular orbital) close to that of the 5f-orbitals of the uranium leading to a significant 5f participation in this orbital.…”