Query related to CVS_EE_SINGLETS in FC-CVS-EOM-EE-CCSD calculation

Questions
#1

Hi
I am trying to do the XAS calculations using FC-CVS-EOM-EE-CCSD method and I got the input example provided in the manual.
But I do not understand what are the numbers in CVS_EE_SINGLETS ([3,0,2,1] in the example)
I understand that its number of excited states in different irrep but how do we decide how many irrep (like 4 here) we have and how to select the numbers?

And also for open shell molecules, what will be the changes required to compute the XAS other than multiplicity of the molecule?

Increasing threshold for oscillator strength
#2

Hello. Can anyone please help in this regard?

#3

Depends on the point group that QChem assigns to your molecule. If it finds the molecule to be Cs then look up the table of irreps for Cs online. As to how each irrep is assigned to the list, I remember reading it somewhere in the manual at some point but couldnt find it.

For open shell molecules use CVS_EE_STATES instead of CVS_EE_SINGLETS or CVS_EE_TRIPLETS. Unless they are core → SOMO transitions, they’ll likely be strongly spin contaminated.

#4

You may find these resources useful:

  1. Notes about symmetry in quantum chemistry calculations: https://iopenshell.usc.edu/resources/howto/symmetry/
  2. A short tutorial lecture and examples of inputs and annotated outputs for various EOM-CC calculations including CVS-EOM-EE/CVS-EOM-IP presented in this workshop. (see sessions 09,10, and 11).

Hope this helps,
Anna.

#5

Hi
I have a follow up question on the same calculation.
So, when I do the fc-CVS-EOM-CCSD calculation in the OSS molecule, there should be transitions from the core to SOMOs but what I am getting are only from core to the virtual orbitals.
For eg. I have 1s as my frozen orbital and I am getting all the excitations from 1s-> virtual orbitals.
Is there any way I can include the excitation from 1s-> SOMOs?
Does increasing the number of EE states help?

Any suggestions would be greatly appreciated.

#6

Hi Rishu,

You should be able to see transitions from core to singly occupied orbitals. Could you post your input file? Please make sure you are looking for EE states with the appropriate symmetry. For example, if your system is C2v and your SOMO has b2 symmetry, then your CCSD state has B2 symmetry and 1s → b2 is a B2 transition in the output, even though the final core-excited state has A1 symmetry.

Core-to-SOMO transition is typically low-lying compared to core-to-virtual transitions from the same core orbital and increasing the number of requested states would not help.

#7

Thanks, Kaushik.
Yes, I am able to see those excited states now. I had some mistake in my previous input.

Another query:
Do you know how can I increase the threshold for printing the oscillator strength from 10^-6(default) to 10^-9?

#8

There is no $rem keyword for increasing the print precision beyond the CCSD energies.