Elasticity of hcp nonmagnetic Fe under pressure

First-principles total-energy calculations of structural and elastic properties of hcp nonmagnetic Fe have been made using the augmented-plane-wave plus local orbital method with the generalized gradient approximation. The equilibrium state, including both the volume and aspect ratio c/a, at a given pressure p is found from the minimum of the Gibbs free energy G with respect to the lattice constants a and c. Abrupt structure is found in c/a below 1000 kbar as a function of p, which agrees with experiment. The elastic constants c(ij) are found from second strain derivatives of G. Particular attention is paid to two corrections to the c(ij), which are evaluated in new ways-a pressure correction required when the c(ij) are evaluated from the energy rather than G and a correction for internal relaxation of the second basis atom in the unit cell. Results are compared with previous first-principles calculations of the c(ij) as functions of pressure.