Maximization of the ratio between Kitaev and residual Heisenberg interactions is a major goal in the current research on Kitaev-Heisenberg magnets. Here we investigate Kitaev-Heisenberg exchange in a recently discovered crystalline phase of $\alpha \text{−}{\mathrm{RuCl}}_3$ under pressure—it displays unusually high symmetry, with only one type of Ru-Ru links and uniform Ru-Cl-Ru bond angles of $\approx {93}^{\circ }$. Using quantum chemical calculations, we find a very small nearest-neighbor Heisenberg $J$, which yields a $K/J$ ratio between Kitaev and Heisenberg exchange as large as $\sim 100$. We also find that this is associated with vanishingly small $d$-shell trigonal splittings, i.e., minimal departure from ideal $j_{\mathrm{eff}}=1/2$ moments. This reconfirms ${\mathrm{RuCl}}_3$ as a most promising platform for materializing the much sought-after Kitaev spin-liquid phase and should stimulate further experiments under strain and pressure.

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• Frustrated magnetism
• Magnetic interactions
• Quantum spin liquid
• Honeycomb lattice
• Configuration interaction
• Kitaev-Heisenberg model
• Quantum chemistry methods