  A new method is given for the model-space effective interaction. Introducing
a new operator in place of the Q-box in the Krenciglowa-Kuo (KK) method, we
derive a new equation for the effective interaction. This equation can be
viewed as an extension of the KK method. We show that this equation can be
solved both in iterative and non-iterative ways. We observe that the iteration
procedure brings about fast acceleration of convergence compared to the KK
approach. We also find that the non-iterative calculation reproduces
successfully any set of the true eigenvalues of the original Hamiltonian. This
non-iterative calculation can be made regardless of the magnitudes of the
overlaps with the model space and the energy differences between the
unperturbed energy and the eigenvalues to be solved.
