The effect of the detailed Raman cross section on the evolution of ult
rashort solitons is considered. It is shown that the gain contribution
leads to the self-frequency shift, whereas the phase contribution, wh
ich is usually determined by use of the Kramers-Kronig relations, has
little visible effect. This result is consistent with the hypothesis t
hat solitons are robust in the presence of Hamiltonian deformations.