Non-self-consistent Density-Functional Theory Exchange-Correlation Forces for GGA Functionals

When using density functional theory (DFT), generalized gradient approximation (GGA) functionals are often necessary for accurate modeling of important properties of biomolecules, including hydrogen-bond strengths and relative energies of conformers. We consider the calculations of forces using non-self-consistent (NSC) methods based on the Harris-Foulkes expression for energy. We derive an expression for the GGA NSC force on atoms, valid for a hierarchy of methods based on local orbitals, and discuss its implementation in the linear scaling DFT code Conquest, using a standard (White-Bird) approach. We investigate the use of NSC structural relaxations before full self-consistent relaxations as a method for improving convergence. Example calculations for glycine and small alanine peptides suggest that NSC pre-relaxations of the structure are indeed useful to save computer effort and time.