GEOMETRY OPTIMIZATION IN AB-INITIO SCF CALCULATIONS - FLOATING ORBITAL GEOMETRY OPTIMIZATION APPLYING THE HELLMANN-FEYNMAN FORCE

Geometry optimizations being still very time-consuming, methods are investigated to save time at least in special cases. The Hellmann-Feynman force is reconsidered as an approximation to the energy gradient and is found useful for the first few iterations of a geometry for molecules having no atoms with polarized inner-shell electrons. The wave-function force" (the difference between the real force and the Hellmann-Feynman force) is utilized finally in a floating orbital geometry optimization to optimize the position of the nuclei relative to the orbital position. This is perfomed simultaneously in the same variable metric procedure as the standard optimization. © 1979."