Unusual band renormalization in the simplest iron-based superconductor<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>FeSe</mml:mtext><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>

The electronic structure of the iron chalcogenide superconductor ${\text{FeSe}}_{1\ensuremath{-}x}$ was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of ${\text{FeSe}}_{1\ensuremath{-}x}$, DFT calculations, and existing data on ${\text{FeSe}}_{x}{\text{Te}}_{1\ensuremath{-}x}$. The bands undergo a pronounced orbital-dependent renormalization, different from what was observed for ${\text{FeSe}}_{x}{\text{Te}}_{1\ensuremath{-}x}$ and any other pnictides.