Particle-particle hole-hole ring diagrams and softening of the nuclear matter equation of state

Nuclear matter isotherms are calculated with the particle-particle hole-hole ring diagrams of the thermodynamic potential included to all orders. A modified Gogny interaction is employed. The resulting isotherms are found to be significantly different from the corresponding HF ones, the former being very much softer. The compression modulus given by our ring-diagram calculation is about 154 MeV. The inclusion of the rearrangement potential due to the ring diagrams plays a crucial role in the present calculation. We evaluate the ring diagrams by way of a finite-temperature RPA equation and there appears to be some definite correlation between the complex eigenvalue of this equation and nuclear matter instability (dp/dρ<0). (orig.)