  The exchange scattering at magnetic adsorbates on superconductors gives rise
to Yu-Shiba-Rusinov (YSR) bound states. Depending on the strength of the
exchange coupling, the magnetic moment perturbs the Cooper pair condensate only
weakly, resulting in a free-spin ground state, or binds a quasiparticle in its
vicinity, leading to a (partially) screened spin state. Here, we use the
flexibility of Fe-porphin molecules adsorbed on a Pb(111) surface to reversibly
and continuously tune between these distinct ground states. We find that the
FeP moment is screened in the pristine adsorption state. Approaching the tip of
a scanning tunneling microscope, we exert a sufficiently strong attractive
force to tune the molecule through the quantum phase transition into the
free-spin state. We ascertain and characterize the transition by investigating
the transport processes as function of tip-molecule distance, exciting the YSR
states by single-electron tunneling as well as (multiple) Andreev reflections.
