atoms gas simulation

int heightPressure;
int atomsAmount = 250;
int atomDiameter = 10;
ArrayList<Atom> atoms = new ArrayList<Atom>();

void setup() {
  size(500, 800);

  heightPressure = height;

  for (int i = 0; i < atomsAmount; i++) {
    atoms.add(new Atom(
      i,
      new PVector(
        random(atomDiameter / 2, width - atomDiameter / 2),
        random(atomDiameter / 2, height - atomDiameter / 2)
      ),
      new PVector(
        random(-1, 1) > 0 ? 1 : -1,
        random(-1, 1) > 0 ? 1 : -1
      ),
      atomDiameter
    ));
  }
}

void draw() {
  background(204);

  heightPressure = mouseY == 0 ? height : mouseY;

  strokeWeight(3);
  line(0, heightPressure, width, heightPressure);
  strokeWeight(6);
  line(width / 2, height, width / 2, heightPressure);
  strokeWeight(1);

  for (Atom atom : atoms) {
    atom.update();
    atom.render();
  }
}

class Atom {
  int atomIndex;
  PVector pos;
  PVector acc;
  int diameter;

  Atom (int index, PVector p, PVector a, int dia) {
    atomIndex = index;
    pos = p;
    acc = a;
    diameter = dia;
  }

  void update() {
    // Width
    if (pos.x > width - diameter / 2) {
      acc.x = -1;
    }
    if (pos.x < diameter / 2) {
      acc.x = 1;
    }

    // Height
    if (pos.y > heightPressure - diameter / 2) {
      acc.y = -1;
      pos.y = heightPressure - diameter / 2;
    }
    if (pos.y < diameter / 2) {
      acc.y = 1;
    }

    if (false) {
      for (int i = 0; i < atoms.size(); i++) {
        if (i != atomIndex) {
          Atom atom2 = atoms.get(i);

          // Width
          if (pos.x > atom2.pos.x - diameter) {
            acc.x = -1;
          }
          if (pos.x < atom2.pos.x + diameter) {
            acc.x = 1;
          }

          // Height
          if (pos.y > atom2.pos.y - diameter) {
            acc.y = -1;
          }
          if (pos.y < atom2.pos.y + diameter) {
            acc.y = 1;
          }
        }
      }
    }

    pos.add(acc);
  }

  void render() {
    ellipse(pos.x, pos.y, diameter, diameter);
  }
}