larger than the electron density [@2012ApJ...749..136L; @2017ApJ...839...22H].

Broadening of H$\alpha$ {#sec:broadening_halpha}
-----------------------

The default Stark broadening mechanism for H$\alpha$ in Multi3D is based on the approximation of @1978JQSRT..20..333S. When the electron density exceeds $10^{13}$ cm$^{-3}$ in the chromosphere, this approximation becomes inaccurate. We tested whether we should include the more accurate unified Stark broadening theory as recently proposed by @2017ApJ...837..125K to model the line profile. We performed 1D tests with several columns from the model atmosphere. For columns with chromospheric electron densities above $10^{13}$ cm$^{-3}$ we indeed found that the line profile computed with the unified theory broadens more. However, most profiles do not show a substantial difference since the typical electron densities are not as high in the line formation region. Since the unified theory is computationally expensive owing to a convolution operation, we decided not to include the unified Stark broadening theory for H$\alpha$.

[H$\alpha$]{} in 1D and 3D {#sec:broadening_halpha}
--------------------------

Previous [H$\alpha$]{} studies using a 3D evaluation of the radiation field were performed using quiet-Sun conditions [@2012ApJ...749..136L]. Under those circumstances, the intensities computed in 1.5D, where the effect of horizontal scattering is neglected, showed a strong imprint of photospheric granulation. A 3D evaluation of the radiation field 