Technique includes acquiring first contact data of first time, associated with first particle in first region; calculating first position data on particles in the first region at second time, and receiving second position data on particles in second region at the second time; detecting second particle being in contact with the first particle and in the first region at the first time and being in the first region at the second time; copying, when the first and second particles are in contact at the second time, displacement of the second particle from the first contact data to second contact data of the second time; detecting third particle being in the first or second region at the second time and in contact with the first particle; and copying, when the third particle is listed in the first contact data, displacement of the third particle to the second contact data therefrom.