Doublet–Quartet Separation and Accurate Fine Structure in the Boron Atom

The energy separation between the doublet and quartet states of the boron atom, together with the fine structure of the quartet manifold, has long remained one of the few unresolved challenges in light-atom spectroscopy and ab initio electronic structure theory. In this work, we employ large-scale expansions of all-particle explicitly correlated Gaussian basis functions and incorporate leading relativistic and quantum electrodynamics (QED) corrections up to order α⁴ to achieve unprecedented accuracy. Our calculations determine the ²P^o–⁴P^e separation in ¹¹B with a sub-0.1 cm^-1 uncertainty, yielding a theoretical excitation energy of 28967.65(7) cm^-1. This result resolves a long-standing ambiguity in the boron spectrum and establishes a new high-precision benchmark for testing many-electron correlation and relativistic effects in open-shell atomic systems. In addition, we predict the fine-structure splittings within the ⁴P^e term, providing valuable guidance for forthcoming laser-spectroscopic measurements aimed at probing this elusive state.