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\"uneisen 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 deep Earth thermodynamic conditions.