Certainly psi4 can generate the mp2 natural orbitals and pass them on to Kallay’s mrcc code. But a line in http://www.psicode.org/psi4manual/master/mrcc.html indicates that rohf references and perturbative methods don’t mix. Is that about right, @jturney? Would uhf-ccsd(t) do for your project?
I don’t know about using natural orbitals in Crawford’s CC code.
I don’t think FNOs are hooked into Crawford’s codes yet. With Kallay’s I haven’t implemented UHF FNOs yet. Actually I just checked in code to throw an exception when this is requested. I don’t it would be difficult to generate UHF FNOs for Kallay’s code. I will try to get this done in the near future.
Well, the OPDM isn’t block diagonal for ROHF-MBPT(2) if you include the singles, right? I guess you could just ignore them - I’m sure the NOs would still have enough information to reliably truncate the virtual space.
Yes, adding singles is trivial, but since the OPDM isn’t block diagonal when you include them, the NOs will mix occupied and virtual orbitals. I think the right way to build the truncated space in this case would be to ignore them so the new virtual NOs won’t have any contributions from the occupied MOs.