  Ionising feedback from massive stars dramatically affects the interstellar
medium local to star forming regions. Numerical simulations are now starting to
include enough complexity to produce morphologies and gas properties that are
not too dissimilar from observations. The comparison between the density fields
produced by hydrodynamical simulations and observations at given wavelengths
relies however on photoionisation/chemistry and radiative transfer
calculations. We present here an implementation of Monte Carlo radiation
transport through a Voronoi tessellation in the photoionisation and dust
radiative transfer code MOCASSIN. We show for the first time a synthetic
spectrum and synthetic emission line maps of an hydrodynamical simulation of a
molecular cloud affected by massive stellar feedback. We show that the approach
on which previous work is based, which remapped hydrodynamical density fields
onto Cartesian grids before performing radiative transfer/photoionisation
calculations, results in significant errors in the temperature and ionisation
structure of the region. Furthermore, we describe the mathematical process of
tracing photon energy packets through a Voronoi tessellation, including
optimisations, treating problematic cases and boundary conditions. We perform
various benchmarks using both the original version of MOCASSIN and the modified
version using the Voronoi tessellation. We show that for uniform grids, or
equivalently a cubic lattice of cell generating points, the new Voronoi version
gives the same results as the original Cartesian-grid version of MOCASSIN for
all benchmarks. For non-uniform initial conditions, such as using snapshots
from Smoothed Particle Hydrodynamics simulations, we show that the Voronoi
version performs better than the Cartesian grid version, resulting in much
better resolution in dense regions.
