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I am a novice in Ehrenfest dynamics and would like to know why Ehrenfest dynamics suffers from the "overcoherence problem"?

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    $\begingroup$ Can you provide more information? A reference for the problem as well as definition of coherence would be useful. $\endgroup$ – Cody Aldaz Jul 25 '20 at 16:58
  • $\begingroup$ Ehrenfest has a lot of problems why use it? Surface hopping and AIMS are much preferable. $\endgroup$ – Cody Aldaz Jul 25 '20 at 17:33
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    $\begingroup$ Ehrenfest is used quite a lot. It strikes a decent balance between accuracy and cost. Vanilla surface hopping doesn't capture quantum coherence properly and AIMS is expensive. This is getting a bit off topic though. The user wants to know why Ehrenfest dynamics suffers from overcoherence. $\endgroup$ – Nike Dattani Jul 25 '20 at 18:09
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Generally speaking, Ehrenfest dynamics tries to model nonadiabatic processes. These are processes where an electronic wavepacket propagates on several potential energy surfaces. The different surfaces have different forces associated to them. This means that the parts of the wavepacket on the different surfaces move in different directions. Ultimately, this leads to decoherence - the parts on the different surfaces do not know about each other anymore.

Ehrenfest dynamics produces only one trajectory that runs on all the potential surfaces in lockstep. This produces an artifical superposition of excited states, which has no possibility for decoherence.

You can start with the discussion around Figure 3 in J. Chem. Theory Comput. 2019, 15, 5031 and work your way up from there.

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