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I'm trying to optimise the excited state of benzene using ORCA 5.0.1 software.

I've already optimised the $S_0$ geometry with CAS-SCF[6,5] with the following input file

! printmos printbasis 6-31g* opt moread

%moinp "opt_rot.gbw"

%casscf
    norb 5
    nel 6
    nroots 3
    mult 1
end

%pal
    nprocs 64
end

* xyzfile 0 1 benzene.xyz

I recall a keyword as iroot or something similar, but I don't recall how to use it and if it's the correct keyword to do what I want to do.

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1 Answer 1

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I see two possibilities here:

  1. you adjust the weights of the roots such that the weight of the s0 root is 0.0 and the weight of the s1 root is 1.0. then you have to hope for the best that the s1 state stays the state that you want and optimize.

  2. if your system has symmetry and the two states have different symmetry, the you can turn on symmetry and optimize the lowest root in the irrep that corresponds to the S1 state.

What is not yet available, is to do a state averaged CASSCF calculation and then do a state specific optimization. I hope that this will come in the future as it will make things like that much easier.

Also, a future project is a root following algorithm in CASSCF. As it stands, you have to keep a keen eye on the optimization that it stays on the surface you want.

Alternatively, if your system is not too exotic or difficult, you can also optimize excited state geometries with TD-DFT. That is actually pretty streamlined and there is a root following algorithm in place.

Chances you are getting a detailed answer are better in the ORCA forum - 50000 users there who are familiar with the program.

Good luck!

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  • $\begingroup$ BTW - iRoot applies to TD DFT, not CASSCF $\endgroup$
    – FrankNeese
    Jan 20, 2023 at 5:38

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