I wander and lost around EOM calculation

I’m trying to calculate(include optimization) a cation radical molecule with EOM-CCSD method.
With a support from Q-chem manual, now I know there are some options for EOM-CCSD(EE/EA/IP/SF/…).
But still I can’t understand about the details such as EA_STATES or EOM_EA_ALPHA/BETA, etc…

I just want to see optimized structure and spin/charge of first excited state.
But it’s too difficult and there are too many options that must be handled, compared to general calculations.

I think my level is not enough to deal with this.
Even so, can you give me a brief advice on input options for reference?
To me, Manual’s descriptions are insufficient(or very difficult) to handle this subject.

Thank you.

Calculations of open-shell species and excited states can be indeed complicated and intimidating. But do not despair!

I suggest to begin with this tutorial on quantum chemistry of open-shell species:

To figure out which keywords to use, you need first to analyze the basic electronic structure of your molecule and its molecular orbitals. What is it – a doublet radical? Which kind? Which states are you interested in?

Let’s assume for the sake of argument that you are interested in the water radical (H2O+) in its ground and excited states. Then you should use neutral closed-shell state of water as a reference (charge=0, multiplicity 1) and specify ionized states (both ground and excited) as IP_STATES (something like [1,0,0,1] – depending on which states you need and what is point group symmetry of your structure). You do not need to worry about IP_ALPHA/BETA keywords, they are not needed in calculations of energy and gradient. If you are planning to optimize your structure, then you also need to specify which EOM-IP state you will compute gradient for.

I recommend to look at the example 7.58 from the online manual (EOM-IP calculations of NO3 radical using closed-shell anionic reference).

Hope this is helpful. If you post details of your system and an example of input, I can provide more specific advice.


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Thank you for your kind advice.

I read tutorial from your link.
And I read another guidance at your homepage.
Now I can pass some obstacles, but I’m still stuck at the IP_STATES(point group symmetry). [n, n, n, n]

My targeted molecule’s point group symmetry is C1 (1 irreducible representation),
and it have resonance structures. With some calculations, I suspects it may have not only monoradical resonance structure but also diradical resonance structure.

I’m looking for the first excited state of cation monoradical(1 2) and it’s structure, excitation energy, charge, spin density through optimization from ground state structure.
In this case, Is it right to write IP_STATES as [1]?
Is there any needs to run Spin Flip options?

Digress from the subject, once I used RAS-SF for the same molecule and I can get some images of NO.
Can I use NO images of RAS-SF to explain spin densities or resonance structures?

Thank you so much.

Yes, for C1 point group you can specify IP_STATES as [1] – this will find the lowest IP state. Note that your reference should be closed shell, spin 1 and charge 0.

I am not sure I understand the diradical part. Diradicals have 2 unpaired electrons, not one. Doublet radical cannot have diradical resonance structures.

You can compute spin densities and natural orbitals for EOM-CC - they are indeed useful for understanding your system.

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