Hello, bonjour, geia sou, shalom. Welcome to the second homework session of Statistical Mechanics: Algorithms and Computations from the Physics Department of the Ecole Normale Superieure. The subject of this week is the simulation of hard disks. The aim is twofold. First: you will convince yourself that the event-driven molecular dynamics gives the same results for thermodynamics as the direct sampling or the Markov chain Monte Carlo methods. You will implement a simple example for each method, compare the output, and verify yourself the famous ergodic hypothesis. Second, you will apply directly this three methods event-driven molecular dynamics, direct sampling and Markov chain to study the phenomenology of hard disks. You will observe that their finite radius leads to unsuspected and surprising phenomenology. For hard disks each legal configuration has exactly the same probability, but do not let this uniform probability distribution mislead you to the assumption that the actual behavior of the system will be uniform and boring. As you will see in this homework, uniform probability distributions can lead to surprisingly non-uniform results. This is due to so called entropic forces which we will study in more details next week. They are everywhere in nature, be assured. In the meantime, have fun with Homework session 2.