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a tutorial on how to select orbitals for active space using Firefly

Valentin Monev

Dear Alex,

I would like to ask you to write a tutorial on how to select orbitals for active space using Firefly.
Thank you in advance!

Please include example input and output files for single reference and multi-reference, as well as state specific and state averaged scenarios, CASSCF as well as XMCQDPT2. How does one recognize that a problem needs to be treated by one of the above scenarios?
Please comment on the input and output files in detail. e.g. the  text in your comments below needs to be expanded upon by pictures of pi- and sigma- orbitals, why are they relevant, how do you recognize a contaminant and when is an orbital free of contaminant, what is a mixed orbital, where are the occupation numbers to be found in the output, should the active space be as wide as possible or restricted...
“My goal was to create pure pi-type active space for this system (although I see that some sigma orbitals are relevant here as well, however, it is outside of the current capabilities of FF to handle such a large active space). Thus, I visualized orbitals and picked up all the pi-type orbitals that were clearly of pi-type. I looked at the occupation numbers as well. Next, not all pi-type orbitals were good enough or free of contaminants. In this case, it is good idea to pick up several additional orbitals that are mixed but have contributions from the conjugated pi-space. This is why the "precas" calculations were necessary at all - the goal was to further "purify" orbitals and get good initial orbitals while reducing the number of orbitals to include into CAS by looking on the occupation numbers. As "precas" uses large active space, the calculations were not actually of CAS type but rather CISDTQ that is sufficient in most cases (at least sufficient for these purposes). Note you need to use GUGA CI for that purpose. After orbitals being additionally localized in "precas" calcs, and active space was truncated, the CAS calculations were performed using ALDET code.”
I have attached a docx file containing snippets from several textbooks, as an introduction to the subject...

Best regards,

This message contains the 15 kb attachment
[ active space.docx ] introduction

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