Marius J. Vilkas

A Fock space coupled cluster study on the electronic structure of the UO2, UO2+, U4+, and U5+ species

The ground and excited states of the UO(2) molecule have been studied using a Dirac-Coulomb intermediate Hamiltonian Fock-space coupled cluster approach (DC-IHFSCC). This method is unique in describing dynamic and nondynamic correlation energies at relatively low computational cost. Spin-orbit coupling effects have been fully included by utilizing the four-component Dirac-Coulomb Hamiltonian from the outset. Complementary calculations on the ionized systems UO(2) (+) and UO(2) (2+) as well as on the ions U(4+) and U(5+) were performed to assess the accuracy of this method. The latter calculations improve upon previously published theoretical work. Our calculations confirm the assignment of the ground state of the UO(2) molecule as a (3)Phi(2u) state that arises from the 5f(1)7s(1) configuration. The first state from the 5f(2) configuration is found above 10,000 cm(-1), whereas the first state from the 5f(1)6d(1) configuration is found at 5,047 cm(-1).

Extrapolated intermediate Hamiltonian coupled-cluster approach: theory and pilot application to electron affinities of alkali atoms.

The intermediate Hamiltonian (IH) coupled-cluster method makes possible the use of very large model spaces in coupled-cluster calculations without running into intruder states. This is achieved at the cost of approximating some of the IH matrix elements, which are not taken at their rigorous effective Hamiltonian (EH) value. The extrapolated intermediate Hamiltonian (XIH) approach proposed here uses a parametrized IH and extrapolates it to the full EH, with model spaces larger by several orders of magnitude than those possible in EH coupled-cluster methods. The flexibility and resistance to intruders of the IH approach are thus combined with the accuracy of full EH. Various extrapolation schemes are described. A pilot application to the electron affinities (EAs) of alkali atoms is presented, where converged EH results are obtained by XIH for model spaces of approximately 20,000 determinants; direct EH calculations converge only for a one-dimensional model space. Including quantum electrodynamic effects, the average XIH error for the EAs is 0.6 meV and the largest error is 1.6 meV. A new reference estimate for the EA of Fr is proposed at 486+/-2 meV.

Electric dipole transitions in ions of the N I isoelectronic sequence

The electric dipole transitions in the nitrogen isoelectronic sequence (Z = 10-30) between the levels of the 1s22s22p3, 1s22s2p4 and 1s22p5 configurations are considered. The stationary second-order many-body perturbation theory (MBPT) is used to account for the electron correlations. Non-local potential is used to calculate the radial part of the effective Hamiltonian. Relativistic corrections were included in the Breit-Pauli approximation. The results include energies, wavelengths, and oscillator strengths for all N-like ions in the interval Z = 10-30.

MBPT CALCULATION OF ENERGY SPECTRA AND E1 TRANSITION PROBABILITIES FOR BORON ISOELECTRONIC SEQUENCE

The energy spectra and the electric dipole transitions in the boron isoelectronic sequence (Z = 8 - 26) between the levels of the 1s22s22p, 1s22s2p2 and 1s22p3 configurations were considered. The stationary second-order many-body perturbation theory (MBPT) was used to account for the electron correlations. The relativistic corrections were included in the Breit-Pauli approximation.

High-accuracy calculations of term energies and lifetimes of silicon-like ions with nuclear charges Z = 24-30

Term energies of the ground and 26 lowest excited levels arising from the 3s23p2, 3s3p3 and 3s23p3d configurations in silicon-like ions with nuclear charges Z = 24–30 are evaluated to high accuracy in relativistic multireference many-body perturbation theory calculations. Theory deviates from experiment on the order of 0.01% for the majority, and by less than 0.2% for all but 13, of the experimentally identified energy levels. Anomalous theory–experiment deviations along the isoelectronic sequence for six excited levels clearly indicate that they have been misidentified, among them the solar lines at 237.61 A and 259.963 A. New identifications for the misidentified levels are given based on theoretical wavelengths and lifetimes. Hitherto unidentified 3s23p3d 3Fo2,3,4 decay lines along the isoelectronic sequence are predicted to well within 0.1 A accuracy. Calculated lifetimes of the odd-parity 3s23p3d 3Fo3 state in silicon-like ions agree reasonably well with experiment, Co13+ being the exception.

Relativistic multireference Møller–Plesset perturbation theory based on multiconfigurational Dirac–Fock reference functions

Abstract Relativistic multireference Moller–Plesset perturbation theory is developed and implemented with analytic basis sets of Gaussian spinors. The theory takes multiconfigurational Dirac–Fock self-consistent field wavefunctions as reference functions. A procedure is described by which to perform multireference second-order Moller–Plesset perturbation theory calculations for a general class of reference functions constructed from one-particle Dirac spinors. Multireference perturbation calculations are reported for the ground and low-lying excited states of oxygen-like iron, Fe 18+ ( Z =26), and ground-state Zn 0 , in which the near degeneracy of a manifold of strongly interacting configurations mandates a multireference treatment.

Relativistic quantum mechanics of many-electron systems

Abstract The present review surveys the single- and multiconfiguration matrix Dirac–Fock self-consistent field methods and their many-body theoretical refinements developed in our group over the last decade. Implementation with analytic basis sets of Gaussian spinors is discussed in detail.

Second-order multiconfigurational Dirac–Fock self-consistent field and multireference configuration interaction calculations on beryllium and beryllium-like Ne6+

Abstract A quadratically convergent Newton–Raphson algorithm for relativistic multiconfigurational Dirac–Fock self-consistent field calculations is developed and implemented with analytic basis sets of Gaussian spinors. A procedure to perform second-order energy optimization for a general class of multiconfigurational wavefunctions constructed from one-particle Dirac spinors is described. We report the results of relativistic multiconfigurational Dirac–Fock self-consistent field calculations on beryllium and beryllium-like neon, followed by multireference configuration interaction with single and double excitations out of the multiconfigurational Dirac–Fock wavefunctions.

Spinel LiMn2−xNixO4 cathode materials for high energy density lithium ion rechargeable batteries

The practical limitations of fully lithium ion insertion and extraction into LiMn2O4 cathode structure without any structural instability make it unsuitable in commercial Li-ion rechargeable batteries. In this work, we showed that those partially substituted by Ni, i.e., LiMn2−xNixO4 (0≤x≤0.5), prepared by sol-gel technique, could be used as a potential candidate for high energy density and high voltage Li-ion battery applications with superior rate capabilities. The improved structural stability of the cathode was probed by x-ray diffraction and micro-Raman spectroscopy. The density-functional theoretical calculations were employed to identify the minimum energy needed for Li+ diffusion pathway and activation energy in the spinel framework with different Ni ion concentrations. Our results showed significant enhancement in the properties with 25at.% of Ni solid-solution doping in LiMn2O4 host and the experimental results are in line with the theoretical computations.

Relativistic multireference many-body perturbation theory calculations on F-, Ne-, Na-, Mg-, Al-, Si- and P-like xenon ions

Many-Body Perturbation Theory (MBPT) has been employed to calculate with high wavelength accuracy the extreme ultraviolet (EUV) spectra of F-like to P-like Xe ions. They discuss the reliability of the new calculations using the example of EUV beam-foil spectra of Xe, in which n = 3, {Delta}n = 0 transitions of Na-, Mg-, Al-like, and Si-like ions have been found to dominate. A further comparison is made with spectra from an electron beam ion trap, that is, from a device with a very different (low density) excitation balance.