Has non-orthogonal CI (NOCI) been implemented in PySCF?

Question

I'm kinda new to Pyscf and was wondering if there is a way to do non-orthogonal CI calculations with it. I would appreciate any help.

Answer 1

NOCI is not available in the main branch of PySCF

That is why your question on the PySCF GitHub page which you asked on the same day as this question (next time please let us know if you are "cross-posting" a question!) was tagged by James Smith with the "feature-request" tag (it means that PySCF does not currently have NOCI as a feature).

If you want to implement NOCI in PySCF by yourself, it will probably be a big task for you if you don't have experience with writing electronic structure software (this is the type of thing that forms a substantial part of masters theses or elite undergrad theses, so it's not something that is done overnight for someone asking a question like this).

If you do want to implement it yourself, the answers on the GitHub page did give you some advice, which I'll summarize here:

• Once you have a way to generate non-orthogonal determinants, you can use this Python interface to libgnme to evaluate non-orthogonal matrix elements, for example $\langle D_i|H | D_j \rangle$ for single-excitations.
• The libgnme GitHub page suggests the following papers for further reading (both by Hugh Burton):
  • Generalized nonorthogonal matrix elements: Unifying Wick’s theorem and the Slater–Condon rules,
  • Generalized nonorthogonal matrix elements. II: Extension to arbitrary excitations.

On the other hand, the free and open source software OpenMolcas can already do NOCI.

Also, after I wrote this answer, it was brought to my attention that someone has indeed implemented NOCI in PySCF but it is not yet merged into the main PySCF branch.