Harmonic and anharmonic properties of Fe and Ni: Thermal expansion, exchange-correlation errors, and magnetism

Abstract

We have investigated the source of errors in ab initio calculations of thermal properties of the magnetic metals Fe and Ni and their dependence on the form of the exchange and correlation functional. We used density-functional theory and density-functional perturbation theory together with the quasiharmonic approximation to compute the coefficient of thermal expansion, bulk modulus and its pressure derivative, phonon modes, and Grueneisen parameters of bcc Fe and fcc Ni. In nonmagnetic metals the main source of error in calculated thermal properties can be attributed to evaluation of properties at incorrect lattice constants, which in turn may be traced to the choice of exchange and correlation functional. However, for magnetic metals the properties may be evaluated at both incorrect lattice constant and incorrect magnetic moment. This affects vibrational properties so that it is no longer true that anharmonic errors are significantly less than errors at harmonic order.