  We present the XMM-Newton RGS and EPIC pn spectra of a long (\simeq 100 ks)
observation of one of the soft X-ray brightest Compton-thick Seyfert 2
galaxies, NGC 424. As a first step, we performed a phenomenological analysis of
the data to derive the properties of all the spectral components. On the basis
of these results, we fitted the spectra with self-consistent photoionisation
models, produced with CLOUDY. The high-energy part of the spectrum is dominated
by a pure neutral Compton reflection component and a neutral iron K-alpha line,
together with K-alpha emission from neutral Ni, suggesting a significant Ni/Fe
overabundance. The soft X-ray RGS spectrum comes mostly from line emission from
H-like and He-like C, N, O, and Ne, as well as from the Fe L-shell. The
presence of narrow RRC from O VIII, O VII, and C VI, the last two with resolved
widths corresponding to temperatures around 5-10 eV, is a strong indication of
a gas in photoionisation equilibrium, as confirmed by the prevalence of the
forbidden component in the O VII triplet. Two gas phases with different
ionisation parameters are needed to reproduce the spectrum with a
self-consistent photoionisation model, any contribution from a gas in
collisional equilibrium being no more than 10% of the total flux in the
0.35-1.55 keV band. When this self-consistent model is applied to the 0.5-10
keV band of the EPIC pn spectrum, a third photoionised phase is needed to
account for emission lines with higher ionisation potential, although K-alpha
emission from S XV and Fe XXVI remains under-predicted.
