Resonance Structures and Input File

How do I differentiate between resonance structures in the input file. For example, is there a way to add formal charge to the atoms of the molecule?


I’m afraid the short answer is “no”. The only molecular inputs to quantum chemical calculations are the positions and types (atomic numbers) of the nuclei, the number of electrons (total charge) and the total electron spin multiplicity (singlet, doublet, etc.). Things like resonance structures and formal charges can be extracted from the output (e.g. the electron density) of the quantum chemical calculation, though there is no unique way to define such properties.

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Why are you interested in doing this? Are you trying to find a particular electronic state or SCF solution? If so, there are some techniques that can accomplish that.

Thanks. I’d be interested in knowing how to extract out the resonance structures, formal charges, and bond order, from the QM calculation.

I’m imagining a basis set where the overall wavefunction for a given molecule is written in terms of its ‘known’ resonance structures. I’d like to understand the contribution each of these resonance forms has to certain properties (in particular polarizability).

It sounds like you’re interested in a qualitative analysis of the results of an electronic structure calculation. There isn’t a uniquely defined way to do this. Deciding ways to do this is its own niche within electronic structure theory. The impression I get is that appropriate vs inappropriate methodology is hotly contested among specialists, but I don’t know the details.

NBO analysis may be of use. Psi already interfaces with it, but it is a paid program. It can do resonance analysis. I don’t know of any other code that supports resonance analysis, but if anybody else does, let us know. If you want to search, the best place to start is probably searching the literature using “Natural Resonance Theory” papers as a starting point.

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Following up on what I found →

Using the NBO software along with FILE47 from Psi4 would then allow me to calculate a wavefunction that represents a particular resonance structure.

However, (apparently) that resulting wavefunction will not be in a format that Psi4 can then use for further calculations (although GAMESS and Guassian would be able to use). Does anyone know if this is indeed the case?

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