Reset of DIIS subspace

Hello everybody,

Recently, I decided to learn more about DIIS method. Programming it on my own, I started getting ill-conditioned equations near the solution. I found an article about DIIS in the Q-chem manual

Here, it is told that resetting the DIIS subspace can help. Could you explain, what it means exactly?

Thank you very much!

The subspace size is how many previous Fock matrices are used in the extrapolation, i.e., it’s the number of j’s in Eq. (4.35) in the link to the manual above.