In 2006, a paper published in "Physics Letters A", concluded that Mills' theoretical hydrino states are unphysical. For the hydrino states, the binding strength increases as the strength of the electric potential decreases, with maximum binding strength when the potential has disappeared completely. The author Norman Dombey remarked "We could call these anomalous states "homeopathic" states because the smaller the coupling, the larger the effect." The model also assumes that the nuclear charge distribution is a point rather than having an arbitrarily small non-zero radius. It also lacks an analogous solution in the Schrödinger equation, which governs non-relativistic systems. Dombey concluded: "We suggest that outside of science fiction this is sufficient reason to disregard them." From a suggestion in Dombey's paper, further work by Antonio Di Castro has shown that states below the ground state, as described in Mills' work, are incompatible with the Schrödinger, Klein–Gordon and Dirac equations, key equations in the study of quantum systems.