Different "SCF" and "Reference" energies for a CCSD calculation?

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Yes, the difference here is due to the CC code defaulting to conventional integrals rather than density fitted. A few comments:

  1. You can set density-fitting on for the CCSD(T) computations with cc_type df in your input options. The density-fitting errors are usually small and the code will be much faster.

  2. Psi4 can handle these focal-point extrapolations combining MP2 and CCSD(T) in an automated way. I’d really recommend doing the calculations with Psi4’s built in handling. The syntax looks like this:
    energy("mp2/aug-cc-pv[tq]z + d:ccsd(t)/aug-cc-pvdz")
    where the d: part indicates the basis you want to take the difference between CCSD(T) and MP2 in. Psi4 can also handle geometry optimizations with the focal-point extrapolations if you want that. Do make sure you’re consistent with the density fitting across SCF/MP2/CCSD(T). See also: Complete Basis Set in the manual.

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