  Crater has shown that, for two particles (with masses $m_1$ and $m_2$) in a
Coulombic bound state, the charge distribution is equal to the sum of the two
charge distributions obtained by taking $m_1\to\infty$ and $m_2\to\infty$
respectively, while keeping the same Coulombic potential. We provide a simple
scaling criterion to determine whether an arbitrary Hamiltonian possesses this
property. In particular we show that, for a Coulombic system, fine structure
corrections preserve this Crater property while two-particle relativistic
corrections and/or hyperfine corrections may destroy it.
