Difference between solar profiles computed with different opacity tabulations providing an estimate of the error coming from opacity modelling. Here, we show the difference in the sound speed for a model computed with our reference opacity table (OP) and a model computed with an alternative opacity (OPAL). Both computations assume standard gravity (𝒴 = 0), and are based on the GS98 metallicity mixture. In our analysis of systematics, the uncertainty in the modelling of opacity was considered constant throughout the star, as explained in Sect. 4. However, one cannot exclude the possibility of a localised opacity variation such as the one emerging from the comparison between OP and OPAL opacity tabulations. For comparison purposes we also show the corresponding impact of the fifth force on the sound speed computed with our reference opacity (OP) and for two values of the fifth force coupling. We discuss the impact of this on inferences of the fifth-force coupling 𝒴 in Sect. 5.