EOM-CC3 calculation

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!

Unfortunately, no. We could add code to do this for CCSD because we already have the two-electron densities, but the CC3 densities are much more complicated, and I don’t have the time to work on it right now.

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