Assuming you want to go this route (and again, make sure there isn’t a better way to compute this number!), Psi should be fine for getting the MOs. I just needed to double-check that before I could give detailed instructions. The post I linked below gives instructions for getting the alpha alpha alpha alpha block of orbitals. I imagine you have UHF wavefunctions, so replace ca with cb to turn an alpha to a beta. The ERIs are integrals and thus not antisymmetrized. They are in chemist’s notation, for orbital indexing.
While I’m at it, I also renamed the topic title, since your question isn’t about the Slater-Condon rules.