Reset of DIIS subspace

Questions
#1

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

https://manual.q-chem.com/5.3/sect_diis.html

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

Thank you very much!

#2

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.