Development of quantum chemical methods for open-shell molecules

The development of high-accuracy quantum chemistry methods for closed-shell singlet molecules in their ground states (like much of organic chemistry) is essentially solved: Coupled-cluster theory provides a hierarchy of methods that quickly and systematically converge to the exact solution (i.e., the full configuration interaction solution). Research in this area is still ongoing in order to extend the reach to larger molecules by constructing lower-scaling approximations.

For a large number of chemical situations that are characterized by an open-shell electronic structure, e.g. transition metal complexes, bond breaking events, or diradicals, the situation is not so clear. The most common methods for treating such situations employ a multiconfigurational complete active space (CAS) reference state and for example correct this wavefunction by 2nd order perturbation theory. Multireference coupled cluster methods (employing a CAS reference) were also developed and give quite accurate results, but are currently limited to very small molecules. Furthermore, increasing the number of active orbitals in the CAS reference state quickly increases the computational cost, since it consists essentially of a full configuration interaction calculation in the active space, which has factorial scaling.

In my laboratory, we aim to develop new quantum-chemical methods that extend the scope and limitations of existing methods. For example, we developed a Hermitian version of the quasidegenerate N-electron valence perturbation theory (HQD-NEVPT2) that addresses problems due to the non-Hermiticity of the QD-NEVPT2 effective Hamiltonian. Furthermore, we are involved in extending the ICE-CI method in ORCA (an approximate full configuration interaction solver for treating large active spaces) to spin-dependent properties.

Related publications:

  • Shubhrodeep Pathak, Lucas Lang and Frank Neese, A dynamic correlation dressed complete active space method: Theory, implementation, and preliminary applications, J. Chem. Phys. 147, 234109 (2017).
  • Lucas Lang and Frank Neese, Spin-dependent properties in the framework of the dynamic correlation dressed complete active space method, J. Chem. Phys. 150, 104104 (2019).
  • Lucas Lang, Kantharuban Sivalingam and Frank Neese, The combination of multipartitioning of the Hamiltonian with canonical Van Vleck perturbation theory leads to a Hermitian variant of quasidegenerate N-electron valence perturbation theory, J. Chem. Phys. 152, 014109 (2020).
  • Lucas Lang, Mihail Atanasov and Frank Neese, Improvement of Ab Initio Ligand Field Theory by Means of Multistate Perturbation Theory, J. Phys. Chem. A 124, 1025 (2020).
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