  Context: Photoevaporation is an important process for protoplanetary disc
dispersal but there has so far been a lack of consensus from simulations over
the mass-loss rates and the most important part of the high-energy spectrum for
driving the wind. Aims: We aim to isolate the origins of these discrepancies
through carefully-benchmarked hydrodynamic simulations of X-ray
photoevaporation with time-dependent thermochemistry calculated on the fly.
Methods: We conduct hydrodynamic simulations with pluto where the
thermochemistry is calculated using prizmo. We explore the contribution of
certain key microphysical processes and the impact of using different spectra
used previously in literature studies. Results: We find that additional cooling
results from the excitation of O by neutral H, which leads to dramatically
reduced mass-loss across the disc compared to previous X-ray photoevaporation
models, with an integrated rate of 10^-9 Msun/yr. Such rates would allow for
longer-lived discs than previously expected from population synthesis. An
alternative spectrum with less soft X-ray produces mass-loss rates around a
factor of 2-3 times lower. The chemistry is significantly out of equilibrium,
with the survival of H2 into the wind aided by advection. This leads to its
role as the dominant coolant at 10s au - thus stabilising a larger radial
temperature gradient across the wind - as well as providing a possible wind
tracer.
