1. 
 Bengtsson, Peter, et al.
(författare)

Energy structure and transition rates in the Nelike sequence from relativistic CI calculations
 2012

Ingår i: Europhysics Conference Abstracts.  European Physical Society.  2914771754 ; :36C

Konferensbidrag (övrigt vetenskapligt)abstract
 Atomic data are important in astrophysical applications and transition rates can be used in the determination of element abundances and plasma diagnostics. To provide for the extensive data needs a number of general computer codes such as SUPERSTRUCTURE, CIV3, and ATSP2K have been developed. As an alternative to these codes, which all rely on the BreitPauli approximation, the fully relativistic GRASP2K code can be used. GRASP2K is based on the multiconfiguration DiracHartreeFock method and implements a biorthogonal transformation method that permits initial and final states in a transition array to be optimized separately, which, in many cases, leads to more accurate values of the resulting rates. The GRASP2K package also contains modules to compute diagonal and offdiagonal hyperfine interaction constants, isotope shifts, Land´e gJ factors, and splittings of magnetic substate in intermediate and strong magnetic fields. In this work, GRASP2K has been applied to provide highly accurate spectroscopic data for ions in the Nelike sequence between Mg III and Kr XXVII. Valence, corevalence, and corecore correlation effects were accounted for through SDMR expansions to increasing sets of active orbitals. In Mg III, Al IV, Si V, P VI, S VII, and Ar IX, for which experimental energies are known to high accuracy, the mean error in the calculated energies is only 0.011%. For ions with no available experimental energy levels the calculated values should be most valuable in various applications. The high accuracy of the calculated energies makes it possible, in some cases, to to point out experimental values that are in error. Babushkin (length) and Coulomb (velocity) forms of transition rates are computed and agree to within a few percent for the majority of the allowed transitions. Computed lifetimes for states belonging to the 2p33s and 2p53d configurations are in good agreement with values from beamfoil measurements as well as from accurate MCHF BreitPauli calculations.


2. 
 Bieron, Jacek, et al.
(författare)

Ab initio MCDHF calculations of electronnucleus interactions
 2015

Ingår i: Physica Scripta.  IOP Publishing.  00318949. ; 90:5

Tidskriftsartikel (refereegranskat)abstract
 We present recent advances in the development of atomic ab initio multiconfiguration Dirac– Hartree–Fock theory, implemented in the GRASP relativistic atomic structure code. For neutral atoms, the deviations of properties calculated within the Dirac–Hartree–Fock (DHF) method (based on independent particle model of an atomic cloud) are usually dominated by electron correlation effects, i.e. the noncentral interactions of individual electrons. We present the recent advances in accurate calculations of electron correlation effects in small, medium, and heavy neutral atoms. We describe methods of systematic development of multiconfiguration expansions leading to systematic, controlled improvement of the accuracy of the ab initio calculations. These methods originate from the concept of the complete active space (CAS) model within the DHF theory, which, at least in principle, permits fully relativistic calculations with full account of electron correlation effects. The calculations within the CAS model on currently available computer systems are feasible only for very light systems. For heavier atoms or ions with more than a few electrons, restrictions have to be imposed on the multiconfiguration expansions. We present methods and tools, which are designed to extend the numerical calculations in a controlled manner, where multiconfiguration expansions account for all leading electron correlation effects. We show examples of applications of the GRASP code to calculations of hyperfine structure constants, but the code may be used for calculations of arbitrary boundstate atomic properties. In recent years it has been applied to calculations of atomic and ionic spectra (transition energies and rates), to determinations of nuclear electromagnetic moments, as well as to calculations related to interactions of bound electrons with nuclear electromagnetic moments leading to violations of discrete symmetries.


3. 
 Bieron, Jacek, et al.
(författare)

Computational Atomic Structure
 2012

Ingår i: Program and Abstracts : Eighth International Conference on Atomic and Molecular Data and Their Applications: ICAMDATA 8.

Konferensbidrag (övrigt vetenskapligt)abstract
 There is an increasing demand for accurate atomic data due to advancements in experimental techniques and investments in large scale research facilities. In astrophysics the quality and resolution of solar and stellar spectra has so improved that the accuracy of atomic data is frequently a limiting factor in the interpretation. Accurate atomic data are also required in plasma physics and in other emerging areas such as laser spectroscopy on isotope separators, Xray lithography, and lighting research. The needs include accurate transition energies, ﬁne and hyperﬁne structures, isotope shifts as well as parameters related to interaction with external magnetic ﬁelds. Also there is a constant need for transition rates between excited states. Data are needed for a wide range of elements and ionization stages. To meet the demands for accurate atomic data the COMPutational Atomic Structure (COMPAS) group has been formed. The group is involved in developing state of the art computer codes for atomic calculations in the nonrelativistic scheme with relativistic corrections in the BreitPauli approximation [1] as well as in the fully relativistic domain. Here we describe new developments of the GRASP2K relativistic atomic structure code [2, 3]. We present results for a number of systems and properties to illustrate the potential and restriction of computational atomic structure. Among the properties are hyperﬁne structures and hyperﬁne quenched rates, Zeeman splittings in intermediate ﬁelds, isotope shifts and transition rates [4]. We also discuss plans for future code developments.


4. 


5. 
 Ekman, Jörgen, et al.
(författare)

Calculations with spectroscopic accuracy : energies, transition rates, and Landé g_Jfactors in the carbon isoelectronic sequence from Ar XIII to Zn XXV
 2014

Ingår i: Astronomy & Astrophysics.  EDP Sciences.  00046361. ; 564

Tidskriftsartikel (refereegranskat)abstract
 Extensive selfconsistent multiconfiguration DiracHartreeFock (MCDHF) calculations and subsequent relativistic configuration in teraction calculations are performed for 262 states belonging to the 15 configurations 2s22p2, 2s2p3, 2p4, 2s22p3l, 2s2p23l, 2p33l and 2s22p4l (l = 0,1,2) in selected carbonlike ions from Ar XIII to Zn XXV. Electron correlation effects are accounted for through large configuration state function expansions. Calculated energy levels are compared with existing theoretical calculations and data from the Chianti and NIST databases. In addition, Landé gJ factors and radiative electric dipole transition rates are given for all ions. The accuracy of the calculations are high enough to facilitate the identification of observed spectral lines.


6. 


7. 
 Froese Fischer, Charlotte, et al.
(författare)

Advanced multiconfiguration methods for complex atoms : I. Energies and wave functions
 2016

Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics.  IOP.  09534075. ; 49:18

Tidskriftsartikel (refereegranskat)abstract
 Multiconfiguration wave function expansions combined with configuration interaction methods are a method of choice for complex atoms where atomic state functions are expanded in a basis of configuration state functions. Combined with a variational method such as the multiconfiguration Hartree–Fock (MCHF) or multiconfiguration Dirac–Hartree–Fock (MCDHF), the associated set of radial functions can be optimized for the levels of interest. The present review updates the variational MCHF theory to include MCDHF, describes the multireference single and double process for generating expansions and the systematic procedure of a computational scheme for monitoring convergence. It focuses on the calculations of energies and wave functions from which other atomic properties can be predicted such as transition rates, hyperfine structures and isotope shifts, for example.


8. 
 Froese Fischer, Charlotte, et al.
(författare)

Configuration interaction with separately optimized pair correlation functions
 2010

Annan publikation (populärvet., debatt m.m.)abstract
 Variational methods produce oneelectron radial functions that minimize the total energy of the system. Independent pair correlation functions (PCFs) designed to represent a specific correlation effect – valence, corevalence, or corecore – can be obtained from multiconfiguration HartreeFock (MCHF) or DiracHartreeFock (MCDHF) calculations [1,2]. These separately optimized and nonorthogonal PCFs may then be coupled by solving the associated generalized eigenproblem. In the present study, the Hamiltonian and overlap matrix elements are evaluated through biorthonormal orbital transformations and efficient countertransformation of the configuration interaction eigenvectors [3]. The ground state of Be atom has been thoroughly tested by this method for various computational strategies and correlation models. It has been shown that the energy convergence is faster than with the usual SDMCHF method of optimizing a single, orthonormal, oneelectron orbital basis spanning the complete configuration space. Beryllium is a small system for which basis saturation can be achieved through complete active space MCHF expansions. But for larger systems describing electron correlation in all space by optimizing a common orthonormal set becomes hopeless whereas the calculation of additional PCFs is straight forward. Our independent optimization scheme, raises many questions related in the choice of the zeroorder model to be used when building the interaction matrix. The present study is the first step in the current development of the extension of the atsp2K and grasp2K packages [1,2] that will adopt the biorthonormal treatment for energies, isotope shifts, hyperfine structures and transition probabilities.


9. 
 Froese Fischer, Charlotte, et al.
(författare)

Doubletquartet energy separation in boron : a partitionedcorrelationfunction interaction method
 2013

Ingår i: Physical Review A.  American Physical Society.  10502947. ; 88:6

Tidskriftsartikel (refereegranskat)abstract
 No lines have been observed for transitions between the doublet and quartet levels of B I. Consequently, energy levels based on observation for the latter are obtained through extrapolation of wavelengths along the isoelectronic sequence for the 2s22p 2Po 3/2 – 2s2p2 4P5/2 transition. In this paper, accurate theoretical excitation energies from a partitionedcorrelationfunctioninteraction (PCFI) method are reported for B I that include both relativistic effects in the BreitPauli approximation and a finite mass correction. Results are compared with extrapolated values from observed data. For B I our estimate of the excitation energy 28 959 ± 5 cm−1 is in better agreement with the values obtained by Edl´en et al. (1969) than those reported by Kramida and Ryabtsev (2007). Our method is validated by applying the same procedure to the separation of these levels in C II.


10. 
 Gaidamauskas, Erikas, et al.
(författare)

Tensorial form and matrix elements of the relativistic nuclear recoil operator
 2011

Ingår i: JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS.  IOP Publishing Ltd.  09534075. ; 44:17

Tidskriftsartikel (refereegranskat)abstract
 Within the lowestorder relativistic approximation (∼v2/c2) and to first order in me/M, the tensorial form of the relativistic corrections of the nuclear recoil Hamiltonian is derived, opening interesting perspectives for calculating isotope shifts in the multiconfiguration Dirac–Hartree–Fock framework. Their calculation is illustrated for selected Li, B and Clike ions. This work underlines the fact that the relativistic corrections to the nuclear recoil are definitively necessary for obtaining reliable isotope shift values.

