Research Focus of Stella Stopkowicz

Current main research interests:

Molecules in strong magnetic fields

Very strong magnetic fields exist on stars like magnetic White Dwarfs (WDs), neutron stars, and magnetars. While on the latter two the magnetic forces completely dominate over Coulomb forces, WDs can have magnetic fields in a range where both forces compete. In such a case, the magnetic field cannot be treated in a perturbative manner as the influence of the field on the electronic structure is simply too strong. We are developing highly-accurate methods (Coupled-Cluster (CC) as well as Equation-of-Motion (EOM) CC) to investigate the properties of atoms and molecules under such conditions.

In the field of astrophysics, observational spectra of many WDs have been collected. However, without accurate theoretical data, assignment of these spectra is not possible. We are therefore aiming for the prediction of highly-accurate theoretical spectra for systems that are expected or known to exist on magnetic WDs.

Artwork of the illustration by Camilla Kottum Elmar.

Press: AIP press release, JGU press release (in German)

Investigation of the magnetic-field- and current-density dependence of the universal density functional

In a collaboration with Prof. A. Teale in Nottingham the universal density functional is investigated numerically using the Lieb variational principle. CC energies and densities computed in the presence of a magnetic field serve as input as well as reference data. Such investigations can be used for the advancement and generation of new density functionals.

Other/previous research interests:

Method development in relativistic quantum chemistry

Development and implementation of:

  • Second-order Direct Perturbation Theory for the description of relativistic corrections for dipole moments, quadrupole moments, and electric field gradients via derivative theory for Hartree-Fock and correlated methods.
  • Fourth-order Direct Perturbation Theory. Here, the perturbative expansion also contains spin-orbit contributions. For Hartree-Fock, the expansion is convergent but was found to diverge for MP2 and CC after the lowest order. The similarity of the perturbative expansion was used to compute spin-orbit contributions as corrections to rigorous scalar-relativistic methods, leading to very accurate results.

Development and implementation of relativistic one- and two-electron integrals

Such integrals are needed for the calculation of energies and electrical properties within Direct Perturbation Theory as well as other relativistic approaches. The implementation is based on the McMurchie-Davidson scheme.

Applications: Prediction of hyperfine parameters and nuclear quadrupole moments

Prediction of nuclear quadrupole moments via highly-accurate quantum chemical calculations including relativistic effects.

Prediction of hyperfine paramaters for the assessment of rotational spectra in collaboration with Prof. C. Puzzarini and Prof. G. Cazzoli.