  We study halo nuclei using a two-frequency shell-model approach employing
wave functions of two different oscillator constants $\hbar\omega_{in}$ and
$\hbar\omega_{out}$, the former for the inner orbits and the latter for the
halo (outer) orbits. An initial application has been made for the halo nuclei
$^6$He and $^6$Li, with $0s_{1/2}$ taken as the inner and ( $0p_{3/2}$,
$0p_{1/2}$) as the halo orbits. Starting from the Paris NN interaction, we have
derived a G-matrix folded-diagram effective interaction for this two-frequency
model space, using an essentially exact treatment for the Pauli exclusion
operator for the G-matrix. While keeping $\hbar\omega_{in}$ fixed, we have
performed calculations with different choices for $\hbar\omega_{out}$, treating
it as a variation parameter. For $\hbar\omega_{in}=19.7 MeV$ and
$\hbar\omega_{out}=8.2 MeV$, our calculated valence energies for $^6$He and
$^6$Li are -2.77 and -3.55 Mev, respectively, both in good agreement with
experiments. The importance of certain three-body-force diagrams is discussed.
