430 Numerical simulations in cosmology
Figure 15.2. Another example of construction of mass refinement in phase space. For
the high-resolution run with box ratios 1:3/3:5/96 the phase space is sampled without
overlapping blocks or gaps.
large distances but degrades the resolution (e.g. Gelb 1992). The PM code is
available (Klypin and Holtzman 1997).
15.2.4.2 The P^3 M code
The P^3 M code is described in detail in Hockney and Eastwood (1981) and
Efstathiouet al(1985). It has two parts: the PM part, which takes care of the
large-scale forces; and the PP part, which adds the small-scale particle–particle
contribution. Because of strong clustering at late stages in the evolution, the PP
part becomes prohibitively expensive once large objects start to form in large
numbers. A significant speed is achieved in a modified version of the code,
which introduces sub-grids (the next levels of PM) in areas with high density