Sascha Bubeck

  • Bubeck
  • Institut für Theoretische Chemie
    Greinstraße 4

    D-50939 Cologne
  • (+49) 221 - 470 - 6886
  • (+49) 221 - 470 - 6896

About Me

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Research Interests

Quantum Chemistry

Simulating chemical systems is the major goal in quantum chemistry. Over the last decades, the advances in computer technology and quantum chemical approaches opened up new possibilities for simulating increasingly larger systems. Computational demands, however, rapidly increase with the size of the system to be simulated. Thus, approximations are required for reducing these demands. Especially the simulation of systems containing heavy atoms is computationally demanding due to the large number of electrons present in such systems. Relativistic effects, which are pronounced in these systems and therefore have to be taken into account, further increase the computational demand.

Effective Core Potentials (Master Thesis)

One approach, which incorporates relativistic effects and reduces computational demands for heavy atoms, is the use of effective core potentials (ECPs) 1. ECPs, of which pseudopotentials (PPs) are a subclass, are suitable for application in various quantum chemical approaches, both for wave function- and density functional-based methods.

One series of PPs, the so-called energy-consistent small-core PPs, were previously generated by M. Dolg et al. for the lanthanide 2 3 and actinide elements 4 5 6 and are used for the majority of simulations of compounds containing these elements.

My master thesis project was focused on the generation of improved PPs of this type for some of the lanthanide elements.

Core Polarization Potentials (PhD Thesis)

ECPs employ the frozen-core approximation, which introduces an error by neglecting the core-core and, more importantly, the core-valence correlation. One way to describe the majority of these neglected correlations is to use an additional operator in the Hamiltonian, the so-called core polarization potential (CPP) operator 7.

This idea dates back to Born and Heisenberg, who corrected their calculated Potassium spectra by accounting for the contribution of the core dipole polarizability 8. After this first semi-empirical application of the CPP operator, Meyer et al. derived the first general theory and applied it to the first molecular systems 9 10. This approach was later used during the generation of energy-consistent large-core PPs for the alkaline elements by Fuentalba et al. 11 12. The method was then refined by Daudey et al., who extended the CPP by a projection operator on the spherical harmonics, making the CPPs dependent on the angular momentum 13. Nowadays, both the l-independent and l-dependent CPP approaches are used, for example for the generation of new energy-consistent di-, tri- and tetravalent PPs by Weigand and Dolg 14 15 16 17 or for the simulation of collision experiments by Gadea et al.18 19. Despite the broad use of the CPP operator in simulations of chemical systems, a systematic investigation of its influence on atomic and molecular properties is still lacking.

Goals of the PhD Thesis:

  • Implementation of CPP routine for generating and using CPP integrals in quantum chemical simulations into QOL
  • Implementation of interface between Hartree-Fock-SCF simulation and Coupled Cluster simulation using the CPP integrals
  • Investigating the influence of the CPP approach in quantum chemical systems


  1. Dolg, M. & Cao, X. Relativistic Pseudopotentials: Their Development and Scope of Applications, Chem. Rev. 112, 403-480 (2012).↩︎

  2. Dolg, M., Stoll, H. & Preuss, H. Energy-adjusted ab initio pseudopotentials for the rare earth elements. J. Chem. Phys. 90, 1730 (1989).↩︎

  3. Dolg, M., Stoll, H., Savin, A. & Preuss, H. Energy-adjusted pseudopotentials for the rare earth elements. Theor. Chim. Acta 75, 173-194 (1989).↩︎

  4. Küchle, W., Dolg, M. Stoll, H. & Preuss, H. Energy-adjusted pseudopotentials for the actinides. Parameter sets and test calculations for thorium and thorium monoxide. J. Chem. Phys. 100, 7535-7542 (May 1994).↩︎

  5. Cao, X., Dolg, M. & Stoll, H. Valence basis sets for relativistic energy-consistent small-core lanthanide pseudopotentials. J. Chem. Phys. 118, 487-496 (2003).↩︎

  6. Cao, X. & Dolg, M. Segmented contraction scheme for small-core actinide pseudopotential basis sets. J. Mol. Struc-Theochem 673, 203-209 (Mar. 2004).↩︎

  7. Dolg, M. & Cao, X. Relativistic Pseudopotentials: Their Development and Scope of Applications, Chem. Rev. 112, 403-480 (2012).↩︎

  8. Born, M. & Heisenberg, W. Über den Einfluß der Deformierbarkeit der Ionen auf optische und chemische Konstanten. I Z. Phys 23, 388-410 (Dec. 1924).↩︎

  9. Müller, W., Flesch, J. & Meyer, W. Treatment of intershell correlation effects in ab initio calculations by use of core polarization potentials. Method and application to alkali and alkaline earth atoms. J. Chem. Phys. 80, 3297-3310 (Apr. 1984).↩︎

  10. Müller, W. & Meyer, W. Ground-state properties of alkali dimers and their cations (including the elements Li, Na, and K) from ab initio calculations with effective core polarization potentials. J. Chem. Phys. 80, 3311 (Apr. 1984).↩︎

  11. Fuentealba, P. On the reliability of semi empirical pseudopotentials: dipole polarisability of the alkali atoms. J. Phys. B: At. Mol. Phys. 15 L555-L558 (1982).↩︎

  12. Fuentealba, P. Die Rolle der Rumpfpolarisierung in Pseudopotentialverfahren. PhD thesis (1982).↩︎

  13. Fourcrault, M. Millie, P. & Daudey, J. P. Nonperturbative method for core-valence correlation in pseudopotential calculations: Application to the Rb2 and Cs2 molecules. J. Chem. Phys. 96, 1257-1264 (Jan. 1992).↩︎

  14. Moritz, A., Cao, X. & Dolg, M. Quasirelativistic energy-consistent 5f-in-core pseudopotentials for divalent and tetravalent actinide elements Theoretical Chemistry Accounts 118, 845-854 (June 2007).↩︎

  15. Weigand, A., Cao, X., Yang, J. & Dolg, M. Quasirelativistic f-in-core pseudopotentials and core-polarization potentials for trivalent actinides and lanthanides: molecular test for trifluorides. Theoretical Chemistry Accounts 126, 117-127 (May 2009).↩︎

  16. Weigand, A., Cao, X., Hangele, T. & Dolg, M. Relativistic Small-Core Pseudopotentials for Actinium, Thorium, and Protactinium. J. Phys. Chem A 118 2519-2530 (2014).↩︎

  17. Weigand, A. Relativistic Energy-consistent Pseudopotentials for f-Elements. PhD thesis (Universität zu Köln, 2009).↩︎

  18. Souissi, H., Mejrissi, L., Habli, H., Al-Ghamdi, A. A., Oujia, B. & Gadéa, F. X., Spectroscopic ab initio investigation of the electronic properties of (SrK)+. Chem. Phys. 490, 19-28 (June 2017).↩︎

  19. Mtiri, S., Mejrissi, Habli, H., Al-Ghamdi, A. A., Oujia & Gadéa, F. X. Theoretical investigation of the diatomic Van der Waals systems Ca+ and CaHe. Comput. Theor. Chem. 1114, 33-46 (Aug. 2017).↩︎