  The three-dimensional Monte Carlo photoionization code Mocassin has been
applied to construct a realistic model of the planetary nebula NGC 3918. Three
different geometric models were tried.
  The effects of the interaction of the diffuse fields coming from two adjacent
regions of different densities were investigated. These are found to be
non-negligible, even for the relatively uncomplicated case of a biconical
geometry. We found that the ionization structure of low ionization species near
the boundaries is particularly affected.
  It is found that all three models provided acceptable matches to the
integrated nebular optical and ultraviolet spectrum. Large discrepancies were
found between all of the model predictions of infrared fine-structure line
fluxes and ISO SWS measurements. This was found to be largely due to an offset
of ~14 arcsec from the centre of the nebula that affected all of the ISO
observations of NGC 3918.
  For each model, we also produced projected emission-line maps and
position-velocity diagrams from synthetic long-slit spectra, which could be
compared to recent HST images and ground-based long-slit echelle spectra. This
comparison showed that spindle-like model B provided the best match to the
observations. We have therefore shown that although the integrated emission
line spectrum of NGC 3918 can be reproduced by all three of the
three-dimensional models investigated in this work, the capability of creating
projected emission-line maps and position-velocity diagrams from synthetic
long-slit spectra was crucial in allowing us to constrain the structure of this
object.
