  Tormen and Bertschinger have presented an algorithm which allows the dynamic
range of N-body simulations to be extended by adding long-wavelength power to
an evolved N-body simulation. This procedure is of considerable interest as it
will enable mock galaxy catalogues to be constructed with volumes as large as
those of the next generation of galaxy redshift surveys. Their algorithm,
however, neglects the coupling between long-wavelength linear modes and
short-wavelength non-linear modes. The growth of structure on small scales is
coupled to the amplitude of long-wavelength density perturbations via their
effect on the local value of the density parameter Omega_0.The effect of
neglecting this coupling is quantified using a set of specially tailored N-body
simulations. It is shown that the large-scale clustering of objects defined in
the evolved density field such as galaxy clusters is strongly underestimated by
their algorithm. An adaptation to their algorithm is proposed that, at the
expense of additional complexity, remedies the shortcomings of the original
one. Methods of constructing biased mock galaxy catalogues which utilise the
basic algorithm of Tormen and Bertschinger, but avoid the pitfalls are
discussed.
