Dear friends,
It is well known that a non-covalent interaction (NCI) could be analyzed through the so-called energy decomposition approach. A given NCI such as H-bond between two interacting monomers could be assessed via supermolecular or SAPT techniques as the two well-know methods while within the supermolecular approach which always needs the BSSE correction characterization of the given interaction is not feasible. It is quite evident that in these analyses all parts (all atoms and bonds) of two monomers are taken into account and not only a specific interaction (such as H-bond) between two interacting systems. In other words, the resulted interaction energy (binding energy) includes the contribution of all atoms within two monomers. The question is that how we are allowed to correlate this energy (as a bulk quantity) to a specific interaction between two monomers such as H-bond.
It’s an approximation and only holds true for small monomers as then the non-bonded contributions of the ‘rest’ of the monomer is small.
Keep in mind that ‘interaction energy decomposition’ is inherently a somewhat arbitrary approach. But it is tremendously helpful to form and understand chemical concepts and phenomena (‘H-bond’ is one such concept).
Dear Holger,
Too many thanks for your prompt reply.
Unfortunately, I did not get your mean properly. Can I sincerely ask to clarify your mean in a more evident manner?
Sincerely,
Saeed
which part is unclear?
Dear Holger,
Please let me explain my understanding from your comments:
Your explanations include any energy decomposition method whether SAPT or supermolecular. On the other hand, with the increase in the size of interacting monomers the validity of such approaches becomes more questionable since the contribution of other interactions between two monomers in the specific interaction we are looking for increase. But I have seen some articles in which these approaches have been applied over the study of interaction energy within big complexes. Consequently, it seems the F-SAPT by which we can center on a specific interaction in a complex should be more reasonable. If so, why F-SAPT is rarely employed for the study of interaction energy in dimers?
Please let me know your valuable opinion and additional comments.
Sincerely,
Saeed
The decomposition remains perfectly valid even for giant monomers. One is just no longer allowed to ascribed the interactions to a small structural subset of the monomer (H-bond, functional groups, etc.). And I don’t think people are doing that.
F-SAPT is both new and only implemented in PSI4. It takes time for people to use it. Some more advertisement (=application papers) from the respective F-SAPT devs could also help 
Personally I dislike a bit the restriction to SAPT0.
Really you made me quite confusing!
I cannot understand your mean. There is inconsistency between two replies. In one reply you said “It’s an approximation and only holds true for small monomers as then the non-bonded contributions of the ‘rest’ of the monomer is small” and in this reply you indicate that “The decomposition remains perfectly valid even for giant monomers”.
Please , if possible, help me to resolve this challenge.
If you want to say: “This H-bond is XX kcal/mol strong” then your monomers must be small.
If your monomers are large all you can say is: "The interaction between these two fragments is XX kcal/mol strong). You cannot pinpoint it to a H-bond alone anymore.
It’s about the interpretation of the results not the method. The method remains physically valid within it’s own definition.
Excellent!
Many thanks for your nice clarification. Please only let me know why the monomers MUST be small if we want to directly judge about the strength of a given H-bond interaction?
And please let me know how many atoms should be interpreted as a small monomer?
No clear answer to how many atoms are ‘small’, that is for you to decide (and for reviewers to agree…).
You basically want to be the H-bond the dominating interacting structural component, the larger the monomer the more contributions from dispersion from the whole monomer you get, even if electrostatic is not an issue.
Too many thanks for your much valuable time to provide highly informative comments.
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