Motivated by the on-going discussion on the nature of magnetism in the quantum Ising chain
CoNb2O6, we present a first-principles-based analysis of its exchange interactions with additional
modeling, addressing drawbacks of a purely density functional theory ansatz. This method allows us
to extract and understand the origin of the magnetic couplings—including all symmetry-allowed terms
- and resolve conflicting model descriptions in CoNb2O6. We find that the twisted Kitaev chain and
transverse-field ferromagnetic Ising chain views are mutually compatible, although additional off-
diagonal exchanges are required for a complete picture. We show that the dominant exchange
interaction is a ligand-centered process—involving eg electrons -, rendered anisotropic by low-
symmetry crystal fields in CoNb2O6, resulting in dominant Ising exchange. Smaller bond-dependent
anisotropies are found to originate from d − d kinetic exchange processes involving t2g electrons. We
demonstrate the validity of our low-energy model by comparing its predictions to measured THz and
INS spectra.

Datasets obtained from exact diagonalization used for INS (Fig5_data) and THz (Fig6_data) plots in the publication.