I am new to Psi4 and would like to calculate adiabatic excitation energies for triplet excited states of medium-sized molecules (6-8 heavy atoms). I can do this with commercial code using CCSD for the ground state and EOM-CCSD for the T1 excited state using a singlet ground-state reference. I would like to continue using the CCSD and EOM-CCSD optimized geometries, but calculate the single-point energies using CC3 and EOM-CC3. It appears this is possible using Psi4, but I’m not quite sure of the memory requirements. Could you provide a rough estimate for the EOM-CC3 single point calculation of the T1 state (say 6 heavy atoms)? Thanks!
Psi4 can provide CC3 ground- and excited-state energies for open-shell systems, but not if you require spin-adapted triplets. The RHF-CC3 code is limited to closed-shell singlets, but, if you’re not too concerned about the exact spin state, the UHF-CC3 code may work for you.
The memory requirements are actually similar to the CCSD code’s requirements because the triples are handled in a batched way to avoid storage of the full list of amplitudes. The UHF-CC3 codes isn’t as well optimized in terms of either memory or CPU as the RHF-CC3 code, but it might still work.