Self‐consistent intermediate Hamiltonians : A coupled cluster type formulation of the singles and doubles configuration interaction matrix dressing

This paper presents a new self‐consistent dressing of a singles and doubles configuration interaction matrix which insures size‐consistency, separability into closed‐shell subsystems if localized molecular orbitals (MOs) are used, and which includes all fourth order corrections. This method yields, among several schemes, a reformulation of the coupled cluster method, including fully the cluster operators of single and double excitations, and partially those of the triples (Bartlett’s algorithm named CCSDT‐1a). Further improvement can be easily included by adding exclusion principle violating corrections. Since it leads to a matrix diagonalization, the method behaves correctly in case of near degeneracies between the reference determinant and some doubles. Due to its flexibility this formulation offers the possibility of consistent combination with less expensive treatments for the study of very large [email protected] ; [email protected]

Improved theory of laser-enhanced ionization in flames: comparison with experiment

An improved theory for laser enhanced ionization in flames has been developed for one‐ and two‐step laser excitations. The model gives an analytical expression for the sensitivity of the method for a given transition of any element. The theoretical expression is compared with experimentally measured signals for a number of elements and the agreement is found to be generally good

Many-body-QED perturbation theory: Connection to the Bethe-Salpeter equation

The connection between many-body theory (MBPT)--in perturbative and non-perturbative form--and quantum-electrodynamics (QED) is reviewed for systems of two fermions in an external field. The treatment is mainly based upon the recently developed covariant-evolution-operator method for QED calculations [Lindgren et al. Phys. Rep. 389, 161 (2004)], which has a structure quite akin to that of many-body perturbation theory. At the same time this procedure is closely connected to the S-matrix and the Green's-function formalisms and can therefore serve as a bridge between various approaches. It is demonstrated that the MBPT-QED scheme, when carried to all orders, leads to a Schroedinger-like equation, equivalent to the Bethe-Salpeter (BS) equation. A Bloch equation in commutator form that can be used for an "extended" or quasi-degenerate model space is derived. It has the same relation to the BS equation as has the standard Bloch equation to the ordinary Schroedinger equation and can be used to generate a perturbation expansion compatible with the BS equation also for a quasi-degenerate model space.Comment: Submitted to Canadian J of Physic

Relativistic quantum dynamics in strong fields: Photon emission from heavy, few-electron ions

Recent progress in the study of the photon emission from highly-charged heavy ions is reviewed. These investigations show that high-$Z$ ions provide a unique tool for improving the understanding of the electron-electron and electron-photon interaction in the presence of strong fields. Apart from the bound-state transitions, which are accurately described in the framework of Quantum Electrodynamics, much information has been obtained also from the radiative capture of (quasi-) free electrons by high-$Z$ ions. Many features in the observed spectra hereby confirm the inherently relativistic behavior of even the simplest compound quantum systems in Nature.Comment: Version 18/11/0

Sequential decoupling of negative-energy states in Douglas-Kroll-Hess theory

Here, we review the historical development, current status, and prospects of Douglas--Kroll--Hess theory as a quantum chemical relativistic electrons-only theory.Comment: 15 page

On Some Attempts to Generalize the Effective Hamiltonian Approach

A clear and simple method to derive various intermediate Hamiltonians is proposed. Several intermediate Hamiltonians usable for various purposes are presented. The new intermediate Hamiltonians have the same properties as the traditional effective Hamiltonians but they should present better convergence properties, and should avoid any intruder state problem

THE NATURE OF ACTINYL-LIGAND BINDING: AnX22−AnX^{2-}_{2} IONS (X=S,Se.Te)(X=S, Se. Te) VALENCE-ISOELECTRONIC WITH AnO22∣AnO^{2|}_{2}

Author Institution: Universit\'{e} P. SabatierThere is substancial current interest in theoretical studies of the actinides, which represent one of the main current challenges for computational chemistry. We have shown that 1-component pseudopotential methods are satisfactory for the uranylion; in particular, DFT methods are highly promising. There does not appear to have been any previous work, either experimental or theoretical, on $AnX^{2+}_{2}$ ions containing the heavier Group 16 elements (S, Se, Te). We find that their electronic and geometrical structures bear little resemblance to those of $AnX^{2+}_{2}$. They have quintet ground states for $An=U$, and are strongly bent, with X-U-X angles of around 50 degres and substantial X-X bonding. As a result of the differences between size and energy of the valence orbitals of S compared to those of O, the predominant bonding interactions in $US^{2+}_{2}$ involve the 6d orbitals on U rather than 5f. While the actinyls are all geometrically similar, the shapes of $US^{2+}_{2}$ and $PuS^{2+}_{2}$ differ appreciably. This observation, together with the novel bonding involved, suggests that these species may have applications as separation agents for the actinides