Continuing the discussion from EOM-CC3 calculation of triplet excited state:

Thank you for the information. My immediate goal is to calculate 0-0 excitation energy for the T1-S0 transition of cyclopentenone in order to assign a spectrum. The conventional wisdom is that you can achieve “chemical accuracy” if you use CC3 for the adiabatic part and a lower level for the ZPVE. I’m just not sure about the error introduced by spin contamination. Could you let me know if Psi4 will calculate the expectation value of S^2? Thanks!