I am performing the frequency calculation for some of my molecules, and gradually optimized the structures till the final optimization using the basis set 6-311+G(d,p) and [opt=tight], then checked for any imag. frequencies, and didn't get any, so I have either local or global minimum. I then performed the stability test and the final output was "The wavefunction is stable under the perturbations considered".
I wonder if the results of the stability test refer to reaching the global min. point, or I should perform the PES scan for each structure?
The stability tests you performed only indicated that you reached a minimum, but this could be either local or global. There are unfortunately) no approaches revealing whether an identified minimum is the global one.
Best,
L.P.
Its even worse.
There is no method to prove that found minimum is global (except of building full PES that is impossible for all not too small molecules). Conformer search normally can help a lot, but one should keep in mind quality of method with wich conformer search is performed.
Stability test makes absolutely other thing - it checks if the wavefunction, that was calculated for given molecule in given geometry is actually best possible. In other words - didnt program accidentially calculate some kind of excited states instead of ground one. Canonical example is RB3LYP calculation of O2. https://www.cup.uni-muenchen.de/ch/compchem/energy/stability.html
Pure organics (HCNO) without exotic functional groups rarely has Stability problem at least in Gaussian (one of few codes that allows this way of tests), for this reason people ignore this check very often even when they start to work with metallocomplexes (or are unaware of this issue).
You can check out ABCluster (http://www.zhjun-sci.com/software.php) and/or Calypso (http://www.calypso.cn/) for finding global minimum. Both can be combined with Gaussian and other packages.
Thank you very much for your reply.
I will try to use the ABCluster to check if I can reach the global min. or more stable conformers than the ones I have now.
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