This is not actually an "error", as I initially thought that it was.  Currently the variables systemEnergyAccumulator and demonEnergyAccumulator are being updated every time that a change is proposed, rather than being updated once per MCS.  Unless the number of MCS is large, these two methods of averaging will yield different results.  The code below shows the changes that will result in data only being accumulated once per MCS.

Alternate method for IdealDemon.java:

  public void doOneMCStep() {
    for(int j = 0;j LESSTHAN N;++j) {
      int particleIndex = (int) (Math.random()*N); // choose particle at random
      double dv = (2.0*Math.random()-1.0)*delta;   // random change in velocity
      double trialVelocity = v[particleIndex]+dv;
      double dE = 0.5*(trialVelocity*trialVelocity-v[particleIndex]*v[particleIndex]);
      if(dE LESSTHANOREQUALTO demonEnergy) {
        v[particleIndex] = trialVelocity;
        acceptedMoves++;
        systemEnergy += dE;
        demonEnergy -= dE;
      }
    }
    systemEnergyAccumulator += systemEnergy;
    demonEnergyAccumulator += demonEnergy;    
    mcs++;
  }


Alternate code for IdealDemonApp.java:

    control.println("Mean Ed = "+idealGas.demonEnergyAccumulator/idealGas.mcs);
    control.println("Mean E = "+idealGas.systemEnergyAccumulator/idealGas.mcs);

Post edited February 21, 2009 at 2:53 PM EST.