Coupled-cluster methods to investigate the chemistry on magnetic white dwarfs

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Most stars in space become white dwarfs (WDs) in the final stage of their lives. Atoms like H, He, C, O, Si, P, S as well as the H2 molecule have been identified in spectra from non-magnetic WDs. In the case of magnetic WDs, which exhibit strong magnetic fields of up to 100 000 Tesla, only H and He have been identified so far. One reason is that because such strong fields alter the electronic spectra so significantly, identifications are solely possible through comparison of the observational spectra to accurate theoretical predictions. Consideration of the magnetic field in the Hamiltonian leads to complex wave functions, thus requiring new implementations of computational methods. So far, only full configuration-interaction calculations have been available, which, due to their enormous computational cost, were only feasible for systems with up to 3-4 electrons. As coupled cluster (CC) and equation-of-motion CC have become standard tools in high-accuracy quantum chemistry, the implementation of these methods for the treatment of atoms and molecules in strong magnetic fields is proposed in order to be able to treat systems beyond the scope of FCI. For the prediction of spectra, excitation energies as well as transition dipole moments are required. Furthermore, consideration of triple excitations is necessary to reach the desired accuracy. For molecules, gradients for ground and excited states are also needed. This set of tools will enable the prediction of magnetic-field dependent spectra for atoms like C, O, Si, etc. Furthermore, due to the use of London orbitals, it also allows to investigate helium clusters as well as molecules like CH, and C2 that may well exist on magnetic WDs. In this way, the proposed project will contribute to unveil the composition of magnetic WD atmospheres.