  Vibrational spectra of long-range molecules are determined accurately and to
arbitrary accuracy with the Canonical Function Method. The energy levels of the
$0^-_g$ and $1_u$ electronic states of the $^{23}{\rm Na}_2$ molecule are
determined from the Ground state up to the continuum limit. The method is
validated by comparison with previous results obtained by Stwalley et al. using
the same potential and Trost et al. whose work is based on the Lennard-Jones
potential adapted to long-range molecules.
