  Head-on collisions of ion-acoustic solitary waves in a collisionless plasma
consisting of cold ions and Boltzmann electrons are studied using the
particle-in-cell simulation. It is shown that the collision of solitary waves
can occur under different scenarios. Solitary waves preserve or do not preserve
their amplitudes and shapes after a collision, depending on their initial
amplitudes. The range of initial amplitudes, at which a solitary wave preserves
its identity after collisions, is established. The use of a diagram of initial
amplitudes of colliding solitary waves to consider possible collision scenarios
is discussed. The characteristic regions in the diagram of the initial
amplitudes corresponding to different collision scenarios are determined, and a
classification of head-on collisions of ion-acoustic solitary waves in a plasma
is proposed.
