Say, a concerted transition state involves the breaking of two bonds and the associated making of two new bonds. Does each bond making/breaking has a corresponding imaginary frequency, so in total there would be two imaginary frequencies. Or, has it only one imaginary frequency which involves both the two breaking/making processes?
Thank you.
A transition state is defined within the Born-Oppenheimer approximation (absent symmetry considerations) as being a first-order saddle point on the molecules nuclear potential energy surface. As such it will always have a single imaginary harmonic frequency associated with it.
From a bonding perspective some of the components of that energy surface will be the state of those two bonds. Moving along the harmonic normal coordinate associated with that imaginary frequency may involve optimal breaking/making of those two bonds, but without knowing the composition of your specific system I don't think that can be said definitively. I suspect it will depend in large part how the other factors contributing to the shape of the energy surface change around the transition state.
Thank you Magnus Hanson-Heine.
What is the meaning of a calculated structure with two imaginary frequencies, each involving a breaking/making process.
Using a state with 2 imaginary frequencies as transition state is not recommendable. Try to re-optimize it with tighter geometry optimization tolerance. (not to be confused with scf-tolerance). It might be helpful to read in the frequencies as a starting information.
Georg Dazinger How exactly can "tighter geometry optimization tolerance" be done? Key words?
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