GDIIS is an acronym for Geometry optimisation using Direct Inversion in the Iterative Subspace. The implemented method is an improved algorithm[1] for large molecules. It is the default optimisation method when the --opt option is specified.
| [1] | Ö. Farkas and H. B. Schegel, Phys. Chem. Chem. Phys. 4, 11 (2002). |