Phosphorous pentoxide is important as a common component of glass and as a reference material for formation enthalpy and free energy of phosphate minerals. Three crystalline polymorphs are well known. Calculating their properties, especially heat capacities, with density functional theory has been difficult. These researchers propose to calculate relative free energies, enthalpies, and heat capacities of the polymorphs of phosphorous pentoxide at standard conditions and at above-ambient pressure and temperature. Earlier results that were obtained with the PBE and PW-91 functionals will be compared to results that follow from the SCAN functional.