  We present realistic models of pion-exchange three-nucleon interactions
obtained by fitting the energies of all the 17 bound or narrow states of 3 <= A
<= 8 nucleons, calculated with less than 2% error using the Green's function
Monte Carlo method. The models contain two-pion-exchange terms due to pi-N
scattering in S- and P-waves, three-pion-exchange terms due to ring diagrams
with one Delta in the intermediate states, and a phenomenological repulsive
term. The models have five parameters: the four strengths and the short-range
cutoff. The 17 fitted energies are insufficient to determine all of them
uniquely.
  We consider five models; they reproduce the observed energies with an rms
error < 1% when used together with the Argonne v18 two-nucleon interaction. In
one of the models the pi-N S-wave scattering interaction is set to zero; in all
others it has the strength suggested by chiral effective field theory. One
model also assumes that the pi-N P-wave scattering interaction has the strength
suggested by effective field theories, and the cutoff is adjusted to fit the
data. In all other models the cutoff is taken to be the same as in the AV18
interaction. The effect of relativistic boost correction is estimated. Results
obtained with only Argonne v8' and v18 interactions are also reported. In
addition, we present results for 7- and 8-body neutron drops in external
potential wells.
