Critical charges of simple Coulomb molecular systems: One-two electron   case
  Let us consider some Coulomb systems of several infinitely massive centers of charge Z and one-two electrons: $(Z,e)$, $(2Z,e)$, $(3Z,e)$, $(4Z,e)$, $(2Z,e,e)$, $(3Z,e,e)$. It is shown that the physical, integer charges $Z=1,2,...$ do not play a distinguished role in total energy and equilibrium distance of a system giving no indication to a charge quantization.   By definition, a critical charge $Z_{cr}$ for a given Coulomb system $(nZ,e)$ or $(nZ,e,e)$ is a charge which separates a domain of the existence of bound states from a domain of unbound ones (continuum). For all above-mentioned systems critical charges $Z_c$ as well as equilibrium geometrical configurations are found. For all studied systems there was obtained an indication to a square-root singularity at $Z=Z_{cr}$. 
A.V. Turbiner, H. Medel Cobaxin
quant-ph math-ph math.MP physics.atom-ph