While there are some numerical thresholds, none of them should give fifth decimal place errors.
Are you sure that these programs are using the same auxiliary basis? I can easily see a disagreement there on the fifth decimal place.
Because CASSCF is variational in all parameters, it’s possible to do a correctness test on the Psi4 energy: check that the dipole that Psi4 reports matches the dipole moment from finite difference of energies. I can run these checks later, but I can’t do that right now.
If anybody else would be willing to take this (see here for an example of finite difference dipoles), I’d appreciate it.
Many thanks! I believe that PSI4 CASSCF code is correct. For the auxiliary basis set, we can find the following strings in the PSI4 output
atoms 1 entry O line 318 file /home/jxzou/software/psi4/share/psi4/basis/def2-universal-jkfit.gbs
atoms 2-3 entry H line 18 file /home/jxzou/software/psi4/share/psi4/basis/def2-universal-jkfit.gbs
And I check the auxbasis data in def2-universal-jkfit.gbs, they are identical. Instead, I just found that if density fitting is also turned on, in the SCF section
for comparison, my previous input here was
then the DF-CASSCF energy is as expected (-75.9350735 a.u.).
This means that, the scf_type pk will affect the mcscf_type df, making the CASSCF energy is neither equal to conventional CASSCF energy, nor equal to DF-CASSCF energy. But ideally, I think we might expect that mcscf_type df would be independent on scf_type pk.