There have been reports of troubled SCF optimizations from within @fevangelista's group. As an attempt to put these to rest, I'll be implementing EDIIS and making DIIS + EDIIS the default. DIIS + EDIIS is a standard SCF convergence technique, used to great effect by a certain code I'm not going to name. Welcome to 2021, Psi4.

Due to potential merge conflicts with #2225, that PR should come in first. Because this week is the target for 1.4, this is slated for 1.5. Pinging @jturney and @susilehtola as other interested parties.