  We introduce a method for solving a self consistent electronic calculation
within localized atomic orbitals, that allows us to converge to the complete
basis set (CBS) limit in a stable, controlled, and systematic way. We compare
our results with the ones obtained with a standard quantum chemistry package
for the simple benzene molecule. We find perfect agreement for small basis set
and show that, within our scheme, it is possible to work with a very large
basis in an efficient and stable way. Therefore we can avoid to introduce any
extrapolation to reach the CBS limit. In our study we have also carried out
variational Monte Carlo (VMC) and lattice regularized diffusion Monte Carlo
(LRDMC) with a standard many-body wave function (WF) defined by the product of
a Slater determinant and a Jastrow factor. Once the Jastrow factor is optimized
by keeping fixed the Slater determinant provided by our new scheme, we obtain a
very good description of the atomization energy of the benzene molecule only
when the basis of atomic orbitals is large enough and close to the CBS limit,
yielding the lowest variational energies.
