HIgher-Order Reduced Density Matrices

[PaulWAyers]

It would be nice to have 3- and 4-electron reduced density matrices, especially (perhaps even exclusively) for the seniority-zero case, where the number of indices in these matrices is very small. This might go together with a special implementation for the DMs of seniority-zero wavefunctions.

I'm tentatively tagging @DanielCalero1 on this, as it may be useful for that project. It isn't a super high priority (yet) though.

[msricher]

There already exists a special implementation of the {1,2}-RDM for seniority-zero ("DOCI") wave functions here.

It is definitely feasible to code the 3 and 4 -RDMs for the DOCI wave function class. Like with the 1 and 2 -RDMs, you just need to determine the unique non-zero blocks${}^{*}$, and then evaluate the elements over all pairs of connected determinants.

${}^{*}$Something like AAA, AAB, ..., BBA, BBB for the 3-RDM, and AAAA, ..., BBBB for the 4-RDM.

[DanielCalero1]

Sounds good. I'll take a look into it!