Dear Turbomole Community,

I am trying to compute the vibrational modes (IR) of an organic cation using DFT e.g. M06-2X and B3LYP // 6-311++G** in Turbomole 7.8.

The closed-shell singlet (CSS) ground state and following excited states, triplet and open-shell singlet (OSS), lying ~0.4 and 0.5 eV higher in energy, were easily optimized within C2v symmetry.

The energetic ordering nicely fits to our NEVPT2//CASSCF calculations and also to experimental photoelectron spectra.

However, there is a vibrational mode in the OSS state (minimum) that is completely wrong, with an unreal frequency and intensity (see below). The mode is a CCC bending and should be below

1300 cm-1. Consequently, the ZPE correction is affected and the ZPE-corrected energy of the OSS state, and relative instability, is shifted to much higher values.

60 a1 3236.52 8.24940 YES YES

61 b1 3242.31 5.36910 YES YES

62 a1 3242.56 9.75005 YES YES

---------- 63 b1 5168.45 325387.74669 YES YES -----------

I also tried to optimize it in C1 symmetry with no success. This extreme frequency appears with both M06-2X and B3LYP. With other functionals the OSS state is a TS.

Would you have any idea of how to proceed?

Thanks in advance,

Dr. Enrique Mendez-Vega

Ruhr-Universität Bochum / OCII