  In this work, the effects of excluded volume are studied in the "one to five
hard disks in a box" system. For one and two disks in different types of cages,
the attractive and repulsive forces are calculated analytically. Attractive
forces are due to excluded volume overlap, and thus allows to understand that
in hard-core systems, second neighbors have an effective interaction, a fact
that is usually neglected when considering only collisions with
first-neighbors. The same effects are observed for numerical computations with
five-disks in a box, as the distributions of distances between disks and disks
to walls suggest. The results indicate that first and second-neighbor excluded
volume interactions are essential to determine the energy landscape topology.
This work supports previous observations that suggests that second-neighbor
excluded volume interactions in hard-core systems are related with phase
transitions.
