For CASSCF,the aim of CEPA/2 calculation is to look at the natural orbitals.In orca manual,benzene molecule is used as a calculation example.I don't know why cas(0,0)：
%moinp "Test-CASSCF-Benzene-1.gbw"
%mrci citype cepa2
tsel 1e-5
natorbiters 1
newblock 1 *
nroots 1
refs cas(0,0) end
end
end
Why is the active space defined at this step?And why (0,0),not other numbers
In CEPA (coupled electron pair approach), single and double excitations are carried out of a reference. CAS(0 electrons, 0 orbitals) in that example simply specifies that the reference is provided by the HF configuration. If you specify other numbers than CAS(0, 0) then your simple CEPA calculation becomes a multireference CEPA calculation where single and double excitations will be carried out with respect to all references coming from the CAS(n, m) setup.
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