I The dispersion interaction and Casimir effect

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The discussion centers on the relationship between dispersion interactions and the Casimir effect, with participants clarifying that the Casimir force is a specific instance of dispersion force. It is noted that density functional theory (DFT) often underestimates dispersion interactions, while Møller–Plesset perturbation theory methods like MP2 may overestimate them. Questions arise regarding whether dispersion interactions are solely attractive and the structural effects on molecules, particularly in hydrocarbons. The importance of citing sources for claims was emphasized, as it aids in providing accurate explanations. Overall, the conversation highlights the complexities of quantum chemistry methods and their implications for understanding intermolecular forces.
Spathi
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TL;DR
I’d like to ask both physicist and physical chemists, whether the dispersion interaction is a kind of Casimir effect (as I heard).
When quantum chemistry computations are performed, different methods are used. Among the most popular are the density functional theory (DFT) methods. And it is known that the DFT approach usually underestimates the dispersion interaction, and to solve this problem, often empirical dispersion corrections are used (e.g. Grimme’s D3). I have also heard that the Møller–Plesset perturbation theory methods (more exactly, a PT addition to the Hartree-Fock method) like MP2 usually overestimate the dispersion interaction. So I’d like to ask both physicist and physical chemists, whether the dispersion interaction is a kind of Casimir effect (as I heard). Also I have a question, is that correct that the dispersion interaction means only the attraction between fragments of molecules; and maybe this is related to a strange form of Ramanujan summation used as a theoretical explanation of the Casimir effect (this sum is negative).
 
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Spathi said:
TL;DR Summary: I’d like to ask both physicist and physical chemists, whether the dispersion interaction is a kind of Casimir effect (as I heard).

as I heard
Where?

You keep doing this - refusing to tell us where you heard something but asking us to explain it to you. Please stop.
 
Vanadium 50 said:
You keep doing this - refusing to tell us where you heard something but asking us to explain it to you. Please stop.

This was a post of a respected member of a chemists forum. Previously I wrote to Dale a PM with explanation why it is sometimes difficult for me to provide such links.
 
Vanadium 50 said:
Where?

You keep doing this - refusing to tell us where you heard something but asking us to explain it to you. Please stop.
As @Spathi mentioned, I am aware of their situation. I recognize that not having the exact source does make answering more difficult. If you are able to help without the details of the source of the question, I would appreciate it.

@Spathi if those details are necessary (sometimes they are), then it simply is a question we will not be able to help you with. That is unfortunate, but it is the best we can do given the situation. I appreciate your understanding in those cases.
 
Yes essentially. The best guide to this is the 2018 book "Fundamentals of van der Waals and Casimir Interactions" by Bo Sernelius. Part III is all about the Casimir effect and dispersion interactions.
 
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Demystifier said:
It's the other way around, Casimir force is a special case of dispersion force. Dispersion force is a force that can be described by a position dependent dispersion relation.
One more question, mostly for quantum chemistry researchers: is the dispersion interaction always the attraction?
I know that linear hydrocarbons like pentane C5H12 have linear structure, but long ones like C21H44 have curved shape, and I have also heard (sorry), that this structural effects is cause by the dispersion (it is reproduced by e.g. B3LYP-D3 method but not by B3LYP). Is this correct? So is the dispersion interaction in molecules is always the attraction?
 
This is a good reason why you should give a good source - the word "London" would have made it clear, to me, anyway, what you were talking about in a way "some guy on the internet said" doesn't.

If you instead wrote "induced dipole-induced dipole" would the answer to your second question be clearer?
 
Spathi said:
is the dispersion interaction always the attraction?
No, not always.
 

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