  While 203 K high temperature superconductivity in H3S has been interpreted by
BCS theory in the dirty limit here we focus on the effects of hydrogen
zero-point-motion and the multiband electronic structure relevant for multigap
superconductivity near Lifshitz transitions. We describe how the topology of
the Fermi surfaces evolves with pressure giving different Lifshitz-transitions.
A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove
singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D
Fermi surface portions appear with slow Fermi velocity where the
Migdal-approximation becomes questionable. We show that the neglected hydrogen
zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an
energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of
type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new
Fermi surfaces appear at the Gamma point of the Brillouin zone here the
Migdal-approximation breaks down and the zero-point-motion induces large
fluctuations. The maximum Tc=203K occurs at 160 GPa where Ef/w0=1 in the small
Fermi surface pocket at Gamma point. A Feshbach-like resonance between a
possible BEC-BCS condensate at Gamma and the BCS condensate in different
k-space spots is proposed.
