Dear fellow psi4 forum-users,

I would like to know if it is possible to get the natural orbitals after a DMRG calculation.

During a calculation (taking for example the “well-known” dmrg-ci example on water) you can find the natural orbital occupation numbers being printed in the stream of the external chemps2 code:

[…]

NOON of irrep A1 = [ 1.99447430135994 , 1.97282427996021 , 0.0252995708859364 , 0.00723392925669578 , 0.000917219043611398 ]

[…]

But unfortunately, I could not find a way to print out what any of these natural orbitals actually look like.

Specifying things like

e_dmrg, wfn_dmrg = psi4.energy(‘dmrg-ci’, return_wfn=‘on’)

will also only return the RHF reference of the dmrg calculation. The option “DMRG_MOLDEN_WRITE” does give pseudocanonical orbitals - but I am not sure how I would re-construct natural orbitals from there, either. Since this “flattened single determinant” comes with fractional occupation numbers, it should not be possible to use a second quantization formalism on them to just set up the one-particle density matrix myself, right? Is there a “workaround” by first projecting onto the canonical orbitals of the RHF reference - i.e. to produce something like a transition density matrix by projecting onto the “well behaved” ground state?

Kind regards and any thoughts appreciated,

fweber