How to extract transition density cube from .fch file in Gaussian?

Question

I have been trying to extract the transition density cube of a simple molecule, $\ce{H_2O}$. I did a regular geometry optimization and then TD-DFT calculation with a keyword Density=Transition=1, which is supposed to write the transition density between the ground and the first excited state in the output, while using Gaussian software. However, if I try to generate the electron density cube in Gaussview from .chk or .fch file in Gaussian Cubegen, the only option available is the SCF density, despite the fact that there is a section "Total CI Rho(1) Density" in the output file. How is it possible to get transition density cube? Should I use additional software like MultiWfn or change my approach to transition density calculation?

Thanks in advance for your help!

Answer 1

In the past I used this tool called Nancy_ex developed by my friend to extract transition density cube directly from the .fchk file. Taken from the description:

An open source code for the analysis of electronic excited states: Natural Transition Orbitals; Detachment and Attachment Density Matrices; Quantum Chemical Charge-Transfer Descriptors. The code is intended to postprocess Gaussian 03 and 09 outputs.

At the link you can find all the information you need. Please let me know if this is what you needed.