Norm of OPDM Calculation

Dear all,

I have a series of dimer configurations and would like to evaluate the norm of the one-particle density matrix (OPDM) to estimate the non-adiabatic coupling (NAC) at each configuration. I see that most of these calculations are carried out using the RAS-2SF method in previously reported literature (J. Phys. Chem. Lett. 2013, 4, 3845−3852, J. Phys. Chem. C 2016, 120, 19070−19077, J. Phys. Chem. A 2021, 125, 875−884). Can the same also be done at the TD-DFT (CAM-B3LYP) level of theory?

In either case, can you please help me out regarding going about this calculation on Q-Chem? Section 7.10.21 in the manual (Q-Chem 5.4) contains commands to compute the norm of the transition OPDM under the $trans_prop input section within the CCSD and EOM-CCSD methods. I wonder if the same input can be used for alternate methods. Is it possible to request Q-Chem to print the norm of the OPDM corresponding to a TD-DFT/RASCI calculation?

I am new to the Q-Chem software and would be extremely grateful for any guidance you can offer.

Thank you for your time and help in advance!

Not sure about RAS-CI, but for TDDFT isn’t the norm of the transition density matrix equal to unity, all the time? Eq. (21) of this review:

In the JPC Lett paper that you cite, they seem to be using norm of an EOM-CCSD wave function to investigate coupling between the locally-excited (LE) states, which are primarily single-excitation character, and the multi-exciton (ME) state, which involves significant double-excitation character. In TDDFT all of the transitions are strictly one-electron excitations, so I’m not sure this makes sense in that case.

Dear Dr. Herbert,

Thank you very much for your reply and for clarifying my query regarding TD-DFT. I am now aware of why alternative methods are used for this calculation and look forward to doing the same.

What I meant about RAS-CI is that I don’t know whether you can get what you want out of Q-Chem or how, but in principle that wave function has doubles in it. Would need to evaluate RAS-CI transition density.

Hi, you can get the OPTDM norm via libwfa if you activate state_analysis = true.

For TDDFT this is implemented as
Omega = ||X||^2 + ||Y||^2
see, Eq. (52) of “Toward an understanding of electronic excitation energies beyond the molecular orbital picture - Physical Chemistry Chemical Physics (RSC Publishing)
If you combine this with John’s equation (||X||^2 - ||Y||^2 = 1), then you find out
Omega = 1 + 2||Y||^2
So, the OPTDM norm tells you something about deexcitations. But I don’t think it has anything to do with NACs.

The paper you cite looks only at a very specific instance of approximating NACs. But it is probably safer to compute the NACs as such. Q-Chem does have this functionality (and other codes as well of course).