lists, and all walls.
%
% The Verlet list of a particle contains other particles that are whithin a
% threshold distance, called Verlet distance, and have a higher ID number.
% This list is updated in a given frequency, which is smaller (less often)
% than the search frequency.
%
% The reference element of each interaction (element 1) is the particle
% with smaller ID number.
%
% For each interaction found, a binary <Interaction.html Interaction> object
% is created to manage the mechanical and / or thermal