Dear Turbomole-Team,

We have recently used Turbomole7.4 in our group in order to generate absorption and emission spectra. For that, we have applied the following procedure:

1) Perform a geometry optimization for the molecule in the electronic ground state (S0)

2) Compute the vertical excitation energy for the transition S0 -> S1

3) Perform a geometry optimization for S1

4) Compute the vertical deexcitation energy for S1 -> S0 transition

5) Perform a normal mode analysis at optimized geometry of S0

6) Perform a normal mode analysis at optimized geometry of S1

Before using the program radless, we ensured that no imaginary frequencies appeared. We then prepared the control and control-gs files (and other files) as described in the user manual. There, we used the following parameters:

$spectral width 4

$delta_t 0.25

$max time 131072

$broadening

$lifetime XXX

where XXX is the lifetime in atomic units (we used values corresponding to up to 40 femtoseconds). In many cases, very good results were achieved with this setup. However, for about 50% of the molecules we have tested, the emission spectrum assumed a shape that appears unreliable to us. We therefore varied the radless-parameters given above. Particulary, we focused on the spectral width and tested values of 1 to 100, but we did not find any improvement. We then checked the geometric displacements listed in the standard output of radless and extracted the maximum value. We find that the larger the displacement the likely radless will result in an unphysical spectrum.

It is not clear to us why these displacements should be so large (values of up to 3000 were observed), because the S1 geometries are not much different from their respective S0 counterparts. We found a hint in the literature that contaminations due to an axis flip can result in large geometric displacements because normal modes of ground and excited states may then be defined in different coordinate systems.

We would like to know if such a contamination can occur in Turbmole's radless program or if this has been compensated for. Are there other reasons I have not mentioned here which may lead to unphysical vibronic spectra?

Thank you in advance for your support.