I have a working copy of Psi4 interfaced with CheMPS2. I have used CheMPS2 as a separate program before (feeding integrals to it), so I know that CheMPS2 is able to disk out 2PDM if requested. I, however, failed to do the same thing with Psi4, and the manual (http://www.psicode.org/psi4manual/master/autodir_options_c/module__dmrg.html) has no information about how to do it. I was wondering if anyone knows an input keyword to make this happen.
Thanks for this. I do not recognize anything listed in column D in the link you mentioned. And I think DMRG_OPDM_AO_PRINT prints out only OPDM. In CheMPS2, I believe there is a separate input rem variable to invoke disking out TPDM. It is definitely possible to disk out TPDM in DMRG, and especially when I compute correlation functions it calculates TPDM. But it just happens that it never disks out TPDM in the end. Thanks very much for investigating this for me.
I am not sure if it would help you solve this issue, but in CheMPS2 if I put --twodmfile=$filename in command line it will save 2PDM into $filename. I suppose a similar functionality exists through Psi4. Any thoughts on this? I appreciate your help.