I've just learned by chance about http://en.wikipedia.org/wiki/Earnshaw_theorem which states that point charges cannot be maintained in a stable stationary equilibrium. But then, what does the negative electric potential energy (in this thread : https://www.physicsforums.com/showthread.php?t=334341) mean? So the crystal wouldn't be stable, but I would need to do some work in order to separate the charges from each other. The only remaining explanation to me is that the charges must oscillate, so there would be a stable dynamic equilibrium. Am I right?