Equation-of-motion coupled-cluster variants in combination with perturbative triples corrections in strong magnetic fields

TL;DR

The paper tackles the challenge of accurately predicting electronic states and spectra of molecules in strong magnetic fields, pertinent to magnetic White Dwarfs, by implementing finite-field variants of equation-of-motion coupled-cluster methods (SF, IP, EA-EOM-CCSD) and nonperturbative triples corrections via CCSD(T)(a)*. It integrates complex ff chemistry with London orbitals to ensure gauge-origin independence and introduces implicit and explicit triples corrections for excited states, enabling reliable treatment of single-excitation-character states in challenging magnetic environments. The authors validate their implementations against ff EE benchmarks and apply them to IPs/EAs of the first two rows, as well as to Na, Mg, Ca, and the CH radical, highlighting Landau-energy contributions and ground-state changes that govern field-dependence and state ordering. The results demonstrate that ff SF-IP/EA-EOM-CCSD(T)(a)* provide spin-pure, symmetry-breaking-free descriptions with substantial accuracy improvements for spectra assignment in magnetic WD atmospheres, while also exposing remaining limitations for states with strong double-excitation character and avoided crossings.

Abstract

In this paper, we report on the implementation of the EOM spin-flip (SF), ionization-potential (IP) and electron-affinity (EA) coupled cluster singles doubles (CCSD) methods for atoms and molecules in strong magnetic fields for energies as well as one-electron properties. Moreover, non-perturbative triples corrections using the EOM-CCSD(T)(a)* scheme were implemented in the finite-field framework for the EE, SF, IP, and EA variants. These developments allow the access to a large variety of electronic states as well as the investigation of IPs and EAs in a strong magnetic field. The latter two indicate the relative stability of the different oxidation states of elements. The increased flexibility to target challenging electronic states and the access to the electronic states of the anion and cation are important for the assignment of spectra from strongly magnetic White Dwarf (WD) stars. Here, we investigate the development of the IPs and EAs in the presence of a magnetic field for the elements of the first and second row of the periodic table. In addition, we study the development of the electronic structure of Na, Mg, and Ca that aided in the assignment of metal lines in a magnetic WD. Lastly, we investigate the electronic excitations of CH in different magnetic-field orientations and strengths, a molecule that has been found in the atmospheres of WD stars.

Figures (9)

• Figure 1: Landau-corrected IPs of the atoms H-Mg in a magnetic field between $0\textnormal{-}0.25\ \textnormal{B}_0$ .
• Figure 2: Landau-corrected EAs of the atoms H-Ne in a magnetic field between 0-0.25 B$_0$.
• Figure 3: Lowest Landau-corrected ionization paths for Na (a) and Mg (b) at the CCSD and CCSD(T)(a)* levels of theory. The corresponding lowest IPs as a function of the magnetic-field strength at the CCSD(T)(a)* level of theory (c).
• Figure 4: Low-lying triplet states of Ca calculated at the SF-EOM-CCSD and SF-EOM-CCSD(T)(a)* levels with unc-aug-cc-pCV5Z basis set.
• Figure 5: The extrapolated B-$\lambda$ curves for the $^3P_u \rightarrow {^3S}_g$ transitions of Ca. The dotted lines correspond to results that assume a simple orbital Zeeman dependence of the energy instead of finite-field predicitions.