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

太空中的大多数恒星在生命的最后阶段都会变成白矮星。在非磁性WDs的光谱中已经鉴定出H、He、C、O、Si、P、S以及H2分子。磁性WDs表现出高达10万特斯拉的强磁场，迄今为止只发现了H和He。其中一个原因是，由于如此强的场对电子光谱的改变如此之大，只有通过将观测光谱与准确的理论预测进行比较，才能进行识别。在哈密顿量中考虑磁场会导致复杂的波函数，因此需要新的计算方法实现。到目前为止，只有完整的构型相互作用计算是可用的，由于它们巨大的计算成本，只能用于多达3-4个电子的系统。随着耦合簇（CC）和运动方程CC已成为高精度量子化学的标准工具，提出了这些方法在强磁场中处理原子和分子的实现，以便能够处理超出FCI范围的系统。对于谱的预测，需要激发态能和跃迁偶极矩。此外，考虑三重激励是必要的，以达到所需的精度。对于分子，也需要基态和激发态的梯度。这套工具将能够预测C、O、Si等原子的磁场依赖谱。此外，由于使用了伦敦轨道，它还允许研究氦团以及可能存在于磁性WDs上的CH和C2分子。通过这种方式，提议的项目将有助于揭示磁性WD大气的组成。

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.