Psi4 writes the pair correlation energies when ‘set pair_energies_print true’ is used.
Can you please let me know, how the pair correlation energies are actually defined and computed in Psi4 (for MP2 and CCSD)?
Next: pair correlation energies are written for singlet states. When I specify a triplet state, pair correlation energies are not written. Why? Can Psi4 give them, too?
Here is my example singlet input:
molecule mol {
0 1
He
}
set scf {
df_scf_guess false
scf_type out_of_core
guess core
}
set ccenergy {
cc_type conv
cachelevel=0
}
set pair_energies_print true
set basis_guess cc-pcvdz
set basis aug-cc-pcvqz
energy(‘ccsd’)
Here is my example triplet input:
molecule mol {
0 3
He
}
set reference rohf
set scf {
df_scf_guess false
scf_type out_of_core
guess core
}
set ccenergy {
cc_type conv
cachelevel=0
}
set pair_energies_print true
set basis_guess cc-pcvdz
set basis aug-cc-pcvqz
energy(‘ccsd’)
Pair energies are only calculated for an RHF reference wavefunction, which explains why triplet Helium doesn’t produce any output. While poking around the code, I noticed that if you add set spinadapt_energies true’ to your singlet input you’ll get singlet and triplet pair energies.
Here is the source code if you’re interested in seeing for yourself: