>Thank you for your honest reply. I also do not know technically how GDIIS works that makes it superior than other methods. Why it is faster and reliable - that I do not know despite that I'm using it. Nevertheless, thanks for the insights!
The original description of the algorithm is :
Pál Császár, Péter Pulay, Geometry optimization by direct inversion in the iterative subspace, Journal of Molecular Structure, Volume 114, 1984, Pages 31-34, ISSN 0022-2860, http://dx.doi.org/10.1016/S0022-2860(84)87198-7.
I do not have access to the article, but you may be able to get it from your institution..