  In this paper we report parallel in situ neutron powder diffraction and Tc(P)
measurements versus pressure on the same MgB2 sample in a He-gas apparatus to
0.6 GPa; in addition, we present Tc(P) measurements in a helium-loaded
diamond-anvil-cell to 20 GPa on the same sample. Our results are compared with
those of other groups. The high precision achieved in these measurements allows
a quantitative evaluation of the change in Tc as a function of the changes in
structure. Our results are consistent with electron-phonon pairing in MgB2.
