Multiwfn forum

aerospace

Member

Registered: 2021-08-17

Posts: 7

Franc-Condon calculation error: low vibronic progression

Dear Dr. Lu,

Recently I have been doing some Franc-Condon calculations in Gaussian, trying to resolve the vibronic spectra of different molecules. However, for some of them, using a PCM model, I have encountered an error that does not occur when doing the calculations in a vacuum:

"ERROR: Low progression after class 2. Total convergence =  3.3%.
        The vibronic spectrum will likely be unreliable. Stopping".

I think the low-frequency modes are causing this issue, as their shift vector is very large; however, experimental resolution is not sufficient to extract them, so I was thinking about omitting them as suggested in this post , however it is not stated explicitly what is the right input setting for RedDim=Block to work. I tried the RedDim=(ClearLowFreq=100) option; however, the calculation leaves only 60 modes out of 200 then.

I would like to inquire about the correct input scheme for the RedDim=Block options: how should it be written? Alternatively, could you suggest any other approaches for calculating the approximate Franck–Condon spectrum in this case with Gaussian software?

I'm quite new to this and do not know how to push this calculation further. Thank you very much in advance for your help!

sobereva

Tian Lu (Multiwfn developer)

From: Athens, Greece

Registered: 2017-09-11

Posts: 2,221

Website

Re: Franc-Condon calculation error: low vibronic progression

This is an example of using redim=block, the first and second modes are ignored.

%oldchk=S0_optfreq.chk
#P freq(FCHT,readfc,readFCHT) geom=check

S0->S1 vibronic absorption

  0  1

reddim=block
Spectrum=HWHM=250

S1_optfreq.chk

1 2