Top panel: comparisons of the ⟨3D⟩ (solid lines) and 1D (dashed lines) atmosphere’s ionisation fractions for the neutral and first two ionisation levels of iron. The [Fe/H] = −2.0 and −3.0⟨3D⟩ atmospheres have been interpolated to produce this plot. Middle panel: ionisation fraction of Fe i for the full 3D (contour) atmosphere for 10 snapshots of the [Fe/H] = −2.0 atmosphere. Contour values represent non-normalised frequencies. Also included are the equivalent ⟨3D⟩ (dash-dot), average 3D (solid line) and 1D LHD (dashed line) ionisation fraction of Fe i. Bottom panel: same as middle panel for the [Fe/H] = −3.0 atmosphere. All ionisation fractions presented in the figure were calculated under the assumption of LTE.