Ok, I think all is working as expected. Consistent with the table, cepa(0) includes singles and is equivalent to lccsd. lccd is equivalent to cepa(0) without singles. By declaring method ocepa(0), you’re entering into a contract that overrides keywords, so no-singles doesn’t work (@hrgrimsl, you were setting as a kwarg, not a keyword anyways).
So the below is what I get, and I think it’s sane. Yes, use lccd.
molecule {
Ne
}
e = energy('lccsd/cc-pvdz')
print(e)
e = energy('lccd/cc-pvdz')
print(e)
e = energy('cepa(0)/cc-pvdz')
print(e)
-128.68046437265727
-128.6802140889028
-128.68046437265727