Simple formula for the atomic forces in the augmented-plane-wave method

We present a simple and explicit method for the computation of atomic forces in ab initio total-energy calculations using a basis of augmented plane waves (APW) and the local-density approximation for exchange and correlation. The force on an atom is given by integrals over its muffin-tin sphere only, which can be obtained easily in existing implementations of the linear APW method, for example. The extra computational cost of calculating the forces on all the atoms is negligible compared with that of performing one single self-consistency iteration step. © 1989 The American Physical Society.