excited states RICC2 – symmetry issue

[Alex_V]

Hi,
I am calculating excited states of planar molecule with ricc2.
When the symmetry is set to Cs I get the electronic transitions of a’ and a” symmetry with particular energies.
After that I need to calculate the normal mode vibrational frequencies for those electronic states, so the symmetry is turned to C1. But now ricc2 results in the electronic states of a-symmetry with the energies of the former a’-type states only, a”-type state solutions are missing.

Have anybody experienced similar symmetry-related problems with ricc2 before?
Any suggestions are highly appreciated.

Regards,
Alex

[Arnim]

Hi Alex,

I have also noticed this behavior. If you have a sequence of excitation 1a', 2a', 1a", 3a',
change the symmetry to C1 and calculate the first 3 excitations in a, the 3a might not
correspond to 1a", but to 3a'.

As far as I understand it, this is due to the fact, that the solution vectors generated
initially before the CCS preoptimization, do not cover this excitation.
You can avoid it, if you choose to create more start vectors, i.e. in this case
$excitations
  irrep=a  nexc=  3  nstart=  4

Cheers,
Arnim

[Alex_V]

Hi Arnim,
thank you for your comments and  suggestions.

>If you have a sequence of excitation 1a', 2a', 1a", 3a',
>change the symmetry to C1 and calculate the first 3 excitations in a, the 3a might not
>correspond to 1a", but to 3a'.

I had exactly the same situation. Increasing the number of start vectors solved this problem,
I increased npre parameter as well.

Thanks again!
Alex

[alex_p]

Hello,

I know the thread is a bit old, but I'm experiencing the same problem. The state in consideration is 1a'. When I calculate 1 and 2 a' and a" and then do the same calculation in c1 symmetry 1a" becomes 1a and 2a" becomes 2a (1a' and 2a' are lower in energy). This problem occurs with aug-cc-pVDZ basis and increasing npre and nstart up to 8 didn't solve the problem. The same calculation works with def2-TZVP basis and increased scfconv and denconv. Is there any possibility to get the calculation working for aug-cc-pVDZ?

kind regards
another alex

[christof.haettig]

When switching off symmetry, I suggest to do first a CIS (or CCS) calculation with a large npre followed by by CIS(D) and compare the
results from this calculation with those from the calculation with symmetry. This will show for how many states one should ask to get
the correct lowest excited states into the reduced space for the Davidson procedure for the solution of the eigenvalue problem.

Cheers,
         Christof