A Coulomb Hamiltonian is a many-particle Schroedinger operator with pair interactions proportional to reciprocal distance. For electrons with two spin states and fixed
distinct nuclei at
with charges
,
its atomic-unit form is
It acts on the antisymmetric space . A fixed nuclear repulsion term may be added
without affecting successive-electron energy differences. Write
and
. Strict binding of the
th electron means
.