  First-principles calculations based on density functional theory have been
widely used in studies of the structural, thermoelastic, rheological, and
electronic properties of earth-forming materials. The exchange-correlation
term, however, is implemented based on various approximations, and this is
believed to be the main reason for discrepancies between experiments and
theoretical predictions. In this work, by using periclase MgO as a prototype
system we examine the discrepancies in pressure and Kohn-Sham energy that are
due to the choice of the exchange-correlation functional. For instance, we
choose local density approximation and generalized gradient approximation. We
perform extensive first-principles calculations at various temperatures and
volumes and find that the exchange-correlation-based discrepancies in Kohn-Sham
energy and pressure should be independent of temperature. This implies that the
physical quantities, such as the equation of states, heat capacity, and the
Gr\"{u}neisen parameter, estimated by a particular choice of
exchange-correlation functional can easily be transformed into those estimated
by another exchange-correlation functional. Our findings may be helpful in
providing useful constraints on mineral properties %at thermodynamic conditions
compatible to deep Earth. at deep Earth thermodynamic conditions.
