  A modified version of the Moller-Plesset approach for obtaining the
correlation energy associated to a Hartree-Fock ground state is proposed. The
method is tested in a model of interacting fermions that allows for an exact
solution. Using up to third order terms improved results are obtained, even
more accurate in the limit of loosely bound particles. This result suggests the
possible convenience of the scheme for the study of chemical bound problems.
