  The isothermal compression of magnesium perovskite and postperovskite is
examined through the F-f plot and the diagnostic plot of Vinet universal model
theoretically from the ab initio quantum-mechanical calculations at the hybrid
Hartree-Fock / Density Functional Theory level. A purely numerical approach,
first time applied in this paper, shows that the discrepancies largely observed
between studies on the perovskite and criticized in geophysical applications
are due to the inadequate choice of the Birch-Murnaghan equation of state;
meanwhile the Vinet model is found utterly appropriate for the mineral and
infers consistent estimations of the bulk modulus and its pressure derivative.
The diagnostics of the postperovskite suggest similar conclusions.
