Select symmetry type (the irrep) of the wave function in CASSCF


I wonder what should be the keyword to select the symmetry type (irrep) in a CASSCF calculation? I have been looking around but failed to find the keyword.


In Psi4, you do this by specifying the complete occupation of your orbitals in your various orbital spaces. You cannot just specify the final electronic state.

See here for an example and here for the possible spaces.

CASSCF gets complicated quickly, so let me know if you need some more advice on how to actually use these keywords.

Sorry, I don’t understand what you mean.
Of course with HF or DFT, you can easily target a state by specifying occupation numbers in each irrep. This is not the case with CASSCF.

Disregard what I had said.

There’s a keyword reference_sym that specifies the symmetry of your CI solutions. 0 is the totally symmetric irrep, and keep counting from there. The canonical ordering of irreps is given here.

Additionally, you can set DOCC and SOCC to control the orbitals of your SCF guess wavefunction. If reference_sym isn’t specified, Psi will default to assuming the symmetry of that wavefunction.

Okay, thanks. This is the keyword I’m looking for.

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