Relativistic electronic-structure calculations employing a two-component no-pair formalism with external-field projection operators

Abstract

A no-pair formalism employing external-field projection operators correct to second order in the potential is used to calculate the 1s energies of one-electron atoms and ground-state properties of the bromine and silver atoms in the framework of the multireference double-excitation configurationinteraction (MRD-CI) method. It is found that the relativistic two-component method that has been used reproduces the one-particle energies of the Dirac equation to order (Za)'. The operator is bounded from below and can be used variationally in relativistic electron-structure calculations of many-electron atoms and molecules. The relativistic correction to the total energy recovers 97% of the relativistic correction of the Dirac-Hartree-Fock (DHF) result in the case of the bromine atom and more than 99%%uo in the case of the silver atom. The relativistic correction of the ionization potential of silver has been calculated to be 0.47 eV at the CI level, in good agreement with DHF results, the correlation contribution in the relativistic case being 0.42 eV. The remaining discrepancy of the absolute value of 6.85 eV (DHF 6.34 eV) to experiment (7.57 eV) is attributed to basis-set deficiencies. The corresponding CI value of the electron affinity (relativistic CI value 1.05 eV, nonrelativistic 0.90 eV) is in much better agreement with experiment (1.30 eV). It is found that correlation contribution and relativistic effects are nonadditive.

Theoretical Characterization of End-On and Side-On Peroxide Coordination in Ligated Cu2O2 Models

“…When BS1d and BS1t were used as basis sets, relativistic effects were accounted for using a Douglas-Kroll-Hess Hamiltonian. 87, 88 The performance of the CASSCF/CASPT2 model, which has been widely used for the quantitative modeling of various aspects of transition-metal chemistry, [89][90][91] depends critically on the choice of the orbital active space for the CAS reference. In our earlier work focusing on the µ-η 2 :η 2 peroxo/bis(µ-oxo) equilibrium, 28 we found that no accessible active space gave a satisfactory description of all structures along the isomerization coordinate.…”

On the reaction of a uranium atom with a nitrogen molecule: a theoretical attempt

“…18 Scalar relativistic effects were included using a Douglas-Kroll (DK) Hamiltonian. 19,20 The effects of spin-orbit (SO) coupling were introduced using a newly developed method based on the CASSCF State Interaction approach (CASSI), 21,22 Here, the CASSCF wave function generated for a number of electronic states are allowed to mix under the influence of a spin-orbit Hamiltonian. The method has recently been described, 16 and we refer to this article for details.…”

Charge Transfer in Uranyl(VI) Halides [UO2X4]2− (X = F, Cl, Br, and I). A Quantum Chemical Study of the Absorption Spectra

“…18,19 All the calculations were performed using the MOLCAS software. 20 In a first step we perform state-average complete active space self-consistent-field calculations (SA-CASSCF).…”