  We introduce a simple generalization of the well known geminal wavefunction
already applied in Quantum Chemistry to atoms and small molecules. The main
feature of the proposed wavefunction is the presence of the antisymmetric
geminal part together with a Jastrow factor. Both the geminal and the Jastrow
play a crucial role in determining the remarkable accuracy of the many-body
state: the former permits the correct treatment of the nondynamic correlation
effects, the latter allows the wavefunction to fulfill the cusp conditions and
makes the geminal expansion rapidly converging to the lowest possible
variational energies. This ansatz is expected to provide a substantial part of
the correlation energy for general complex atomic and molecular systems. The
antisymmetric geminal term can be written as a single determinant even in the
polarized cases. In general, therefore, the computational effort to sample this
correlated wavefunction is not very demanding. We applied this Jastrow-geminal
approach to atoms up to Z=15, always getting good variational energies, by
particularly improving those with a strong multiconfigurational nature. Our
wavefunction is very useful for Monte Carlo techniques, such as Fixed node.
Indeed, the nodal surface obtained within this approach can be substantially
improved through the geminal expansion.
