I am implementing from scratch an Hartree-Fock calculation in the STO-3G basis set to perform Born-Oppenheimer molecular dynamics. I have a Restricted Hartree-Fock procedure that can reproduce very well the total energies of Ref. [1] and of Gaussian09 with pre-optimized molecular geometries.
Now, during a Born-Oppenhaimer MD simulation, I will have geometries that can be quite far from equilibrium. I saw that my SCF cycle does not converge for stretched geometries: the SCF cycle for the $CO$ molecule with a bond length of 2.132 bohr converges, while for a bond length of 3.132 bohr I reach the maximal number of allowed iterations (500).
In Table 10.4 of Ref. [2] it is clear that an implementation of the DIIS (Direct Inversion of the iterative Subspace) method speedup the SCF convergence. In addition to this speedup, it seems that the convergence for a stretched molecule can be easily reached with this method. For this reason I implemented the DIIS algorithm following Ref. [3].
I believe I implemented the DIIS method quite correctly, since I was able to obtain a speedup of SCF convergence for pre-optimized molecular geometries. In the following table you can find the number of simple SCF and SCF-DIIS steps I have, for different molecules:
Molecule SCF DIIS
H2O 16 9
CO 53 19
HeH+ 8 7
CH4 11 8
FH 10 7
O2 41 18
N2 110 22
In every molecule I tested I get a speedup, which is quite impressive where a lot of normal SCF cycles were needed. This is a good result, but unfortunately the convergence of stretched molecules is not improved. By doubling the distances (in bohr) of the pre-optimized structure, the DIIS algorithm (as well as the normal SCF) fails to converge.
There is some step to add to the original DIIS algorithm of Ref. [3] that I am not considering? How I can make the SCF cycle convergent for stretched molecules, provided that these calculations converge in Gaussian09?
EDIT
Gaussian09 brakes down as my program does with the option SCF=NoDIIS
. With the option SCF=DIIS
the SCF converges even for very distorted molecules.
[1] A. Szabo and N. Ostlund, Modern Quantum Chemistry, Dover, 1996.
[2] T. Helgaker, P. Jørgensen and J. Olsen, Molecular Electronic-Structure Theory, Wiley, 2000.
[3] P. Pulay, Improved SCF Convergence Acceleration, Journal of Computational Chemistry, 1982.