SOFT 2-MESSON-EXCHANGE NUCLEON-NUCLEON POTENTIALS .2. ONE-PAIR AND 2-PAIR DIAGRAMS

Two-meson-exchange nucleon-nucleon potentials are derived where either one or both nucleons contains a pair vertex. Physically, the meson-pa ir vertices are meant to describe in an effective way (part of) the ef fects of heavy-meson exchange and meson-nucleon resonances. From the p oint of view of ''duality,'' these two kinds of contribution are rough ly equivalent. The various possibilities for meson pairs coupling to t he nucleon are inspired by the chiral-invariant phenomenological Lagra ngians that have appeared in the literature. The coupling constants ar e fixed using the linear sigma model. We show that the inclusion of th ese two-meson exchanges gives a significant improvement over a potenti al model including only the standard one-boson exchanges.