  In this review we first discuss extension of Bohr's 1913 molecular model and
show that it corresponds to the large-D limit of a dimensional scaling
(D-scaling) analysis, as developed by Herschbach and coworkers.
  In a separate but synergetic approach to the two-electron problem, we
summarize recent advances in constructing analytical models for describing the
two-electron bond. The emphasis here is not maximally attainable numerical
accuracy, but beyond textbook accuracy as informed by physical insights. We
demonstrate how the interplay of the cusp condition, the asymptotic condition,
the electron-correlation, configuration interaction, and the exact one electron
two-center orbitals, can produce energy results approaching chemical accuracy.
Reviews of more traditional calculational approaches, such as Hartree-Fock, are
also given.
  The inclusion of electron correlation via Hylleraas type functions is well
known to be important, but difficult to implement for more than two electrons.
The use of the D-scaled Bohr model offers the tantalizing possibility of
obtaining electron correlation energy in a non-traditional way.
