Multiple Δ(3, 3) excitation and the binding energy of nuclear matter

The contributions of ring diagrams to the binding energy of nuclear matter are evaluated. A technique is developed which allows the numerical calculation of third- and fourth-order rings including isobar excitations and using realistic interactions. It is found that ring diagrams involving isobar excitation are of similar importance as the corresponding conventional pure nucleonic terms. The contribution of the ring diagrams is reduced if a nucleon-nucleon interaction is used with a weak tensor force.