Is it possible to print the 2-RDM with Q-Chem?

Hi everyone,

I’m interested in obtaining the 2-electron reduced density matrix (2-RDM), as they appear (in the natural orbital basis) in the equations for symmetry-adapted perturbation theory.

Is there an option to print the 2-RDM for a chosen method? For example, could I request Q-chem to print the 2-RDM that corresponds to a state in a TD-DFT or CASSCF calculation?

In case this is not possible for a general method, what about for the v2RDM method?

Thank you for your time.


I am not sure about v2RDM or CASSCF but for most many-body methods the answer is no, because in most cases the wave function is never actually constructed explicity.

I am a little confused what you mean actually in the case of TDDFT (which is performed entirely using 1-electron density matrices) and SAPT (where the 2-RDM does not appear explictly; like most perturbation theories everything is done in terms of amplitudes contracted with integrals).

Thank you for your answer.

It’s good to know that in general the 2-electron density isn’t available.

In regards to SAPT, I have seen the 2-RDM appear in a multi-configurational formulation of SAPT described in this paper The reason I asked about the 2-RDM is because I am interested in using it with this variant of SAPT to calculate interaction energies.

The multiconfigurational version of SAPT is very new and if you read the fine print of that paper you will find that it is only implemented “locally”, in an unspecified code. It is not available in Q-Chem and there are no plans to do so.