  We show that a simple correlated wave function, obtained by applying a
Jastrow correlation term to an Antisymmetrized Geminal Power (AGP), based upon
singlet pairs between electrons, is particularly suited for describing the
electronic structure of molecules, yielding a large amount of the correlation
energy. The remarkable feature of this approach is that, in principle, several
Resonating Valence Bonds (RVB) can be dealt simultaneously with a single
determinant, at a computational cost growing with the number of electrons
similarly to more conventional methods, such as Hartree-Fock (HF) or Density
Functional Theory (DFT). Moreover we describe an extension of the Stochastic
Reconfiguration (SR) method, that was recently introduced for the energy
minimization of simple atomic wave functions. Within this extension the atomic
positions can be considered as further variational parameters, that can be
optimized together with the remaining ones. The method is applied to several
molecules from Li_2 to benzene by obtaining total energies, bond lengths and
binding energies comparable with much more demanding multi configuration
schemes.
