private boolean overlap(GayBerneParticle p) {
		if (p == null)
			throw new IllegalArgumentException("p is null");
		for (int i = 0; i < nParticle; i++) {
			if (gb[i] != p) {
				if (gb[i].intersects(p))
					return true;
			}
		}
		return false;
	}