  In computer simulations performed in constant number of particles ensembles,
although the total number of particles N contained in the simulation box does
not fluctuate, hence giving a zero apparent compressibility, there are still
local fluctuations in the number of particles. It is shown herein that these
apparent fluctuations produce a compressibility that can be computed from the
calculated radial distribution function, and which matches to a great accuracy
the compressibility of the fluid for the open system. This statement implies
that the radial distribution function evaluated in simulation of constant
number of particles is identical to that evaluated in the grand canonical
ensemble, for the entire distance range within half-box width. This is
illustrated for the hard sphere and Lennard-Jones fluids and for molecular
models of water. The origin of this apparent fluctuation is that the bulk of
the remaining particles, outside the range over which the distribution function
is calculated, act as a reservoir of particles for those within this range,
thanks to the periodic boundary conditions. The implications on the calculation
of the Kirkwood-Buff integrals are discussed.
