Tamfang It's easy to imagine the electron repulsion algorithm. From the shapes of the holes I guess each hole represents a charge; but that means some of the charges settled on the crossing circle, which I'd expect them to avoid; the charges are effectively twice as dense there as elsewhere. So: Is my imagination wrong? Is my understanding of the model wrong? Did you pin some of the charges to the crossing? Did you run the program many times and pick one with a good crossing?