I do not understand your question.<br /><br />The interaction force between any two particles are model with relation: <img class='tex' src="/demolab/smf/teximages/4/9/d/49d749c735434a83fe16ff687c5a9e79.png" alt="\vec{F}=-\frac{GMm}{r}\hat{r}" /> where <img class='tex' src="/demolab/smf/teximages/4/b/4/4b43b0aee35624cd95b910189b3dc231.png" alt="r" /> is the distance between any two particles. (because it is 2D model so force is proportional to 1/r).<br />The gravitationf force should be model with <img class='tex' src="/demolab/smf/teximages/c/2/2/c22ca61813d725fa0dacf3ede3bdd60e.png" alt="\vec{F}=-\frac{GMm}{r^2}\hat{r}" /> for 3D simulation.<br />And particles are generated at random position with random velocity.