FORMATION ENERGY AND LATTICE-RELAXATION FOR POINT-DEFECTS IN LI AND AL

Calculations were performed for both a vacancy and an Al solute atom in bcc Li and for a vacancy in fcc Al. The purpose of this work was (i) to test optimization algorithms that allow a unified determination of ground-state electronic structure and lattice relaxation, and (ii) to compare calculated properties with experiment and with previous pair-potential simulations. 16- and 54-site supercells were employed in the Li-host calculations and a 32-site supercell was employed for the Al vacancy. The self-consistent Kohn-Sham orbitals, expanded in a plane-wave basis, were obtained using the modified steepest-descents algorithm of Williams and Soler and the band-by-band iteration method of Teter, Payne, and Allan. Electron-ion interactions were represented by generalized norm-conserving pseudopotentials cast in separable form. The relative performance of the two optimization algorithms is discussed. The equilibrium lattice relaxation was calculated by the Newton-Raphson method, with the Hessian matrix determined from numerical derivatives of the Hellman-Feynman forces. Calculated vacancy-formation energies are in excellent agreement with experiment.