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Observational breakthroughs in the exoplanet field of the last decademotivated the development of numerous theoretical models describing atmospheresand mass loss, which is believed to be one of the main drivers of planetaryevolution. We aim to outline for which types of close-in planets in theNeptune-mass range the accurate treatment of photoionisation effects is mostrelevant concerning atmospheric escape and the parameters relevant forinterpreting observations. We developed the CHAIN (Cloudy e Hydro AncoraINsieme) model combining 1D hydrodynamic upper atmosphere model with thenon-LTE photoionisation and radiative transfer code Cloudy accounting forphotochemistry, detailed atomic level populations, and chemical reactions forall elements up to zinc. We apply CHAIN to model the upper atmospheres of arange of Neptune-like planets with masses between 1 and 50 M$_{\oplus}$,varying also the orbital parameters. For the majority of warm and hot Neptunes,we find slower and denser outflows, with lower ion fractions, compared to thepredictions of the hydrodynamic model alone. Furthermore, we find significantlydifferent temperature profiles between CHAIN and the hydrodynamic model alone,though the peak values are similar for similar atmospheric compositions. Themass-loss rates predicted by CHAIN are higher for hot, strongly irradiatedplanets and lower for more moderate planets. All differences between the twomodels are strongly correlated with the amount of high-energy irradiation.Finally, we find that the hydrodynamic effects impact significantly ionisationand heating. The impact of the precise photoionisation treatment provided byCloudy strongly depends on the system parameters. This suggests that some ofthe simplifications typically employed in hydrodynamic modelling might lead tosystematic errors when studying planetary atmospheres, even at apopulation-wide level.

Precise photoionisation treatment and hydrodynamic effects in atmospheric modelling of warm and hot Neptunes