  Radiation hydrodynamics (RHD) simulations are used to study many
astrophysical phenomena, however they require the use of simplified radiation
transport and thermal prescriptions to reduce computational cost. In this paper
we present a systematic study of the importance of microphysical processes in
RHD simulations using the example of D-type HII region expansion. We compare
the simplest hydrogen-only models with those that include: ionisation of H, He,
C, N, O, S and Ne, different gas metallicity, non-LTE metal line blanketed
stellar spectral models of varying metallicity, radiation pressure, dust and
treatment of photodissociation regions. Each of these processes are explicitly
treated using modern numerical methods rather than parameterisation. In line
with expectations, changes due to microphysics in either the effective number
of ionising photons or the thermal structure of the gas lead to differences in
D-type expansion. In general we find that more realistic calculations lead to
the onset of D-type expansion at smaller radii and a slower subsequent
expansion. Simulations of star forming regions using simplified microphysics
are therefore likely overestimating the strength of radiative feedback. We find
that both variations in gas metallicity and the inclusion of dust can affect
the ionisation front evolution at the 10-20 per cent level over 500kyr, which
could substantially modify the results of simplified 3D models including
feedback. Stellar metallicity, radiation pressure and the inclusion of
photodissociation regions are all less significant effects at the 1 per cent
level or less, rendering them of minor importance in the modelling the
dynamical evolution of HII regions.
