  A significant fraction of unstable multiple planet systems likely scatter
during the transitional disc phase as gas damping becomes ineffectual. Using an
ensemble of FARGO hydrodynamic simulations and MERCURY n-body integrations, we
directly follow planet-disc and planet-planet interactions through the clearing
phase and on through 50 Myr of dynamical evolution. Disc clearing occurs via
X-ray driven photoevaporation. The hydrodynamic evolution of individual
scattering systems is complex, and involves phases in which massive planets
orbit within eccentric gaps, or accrete directly from the disc without a gap.
Comparing the results to a gas-free model, we find that the n-body dynamics and
hydrodynamics of scattering into one- and two-planet final states are almost
identical. The eccentricity distributions in these channels are almost
unaltered by the presence of gas. The hydrodynamic simulations, however, also
form low eccentricity three-planet systems in long-term stable configurations,
and the admixture of these systems results in modestly lower eccentricities in
hydrodynamic as opposed to gas-free simulations. The incidence of these
three-planet systems is likely a function of the initial conditions; different
planet setups (number or spacing) may change the character of this result. We
analyze the properties of surviving multiple planet systems, and show that only
a small fraction (a few percent) enter mean-motion resonances after scattering,
while a larger fraction form stable resonant chains and avoid scattering
entirely. Our results remain consistent with the hypothesis that exoplanet
eccentricity results from scattering, though the detailed agreement between
observations and gas-free simulation results is likely coincidental. We discuss
the prospects for testing scattering models by observing planets or
non-axisymmetric gas structure in transitional discs.
