Is there a reliable way to determine if a chemical system is more suited to be calculated using a multireference method instead of DFT?

Some time ago I recall hearing a professor say that looking at a DFT result (I don't remember exactly what, a TD-DFT, spin contamination etc...), he could tell that the system was multireferenced.


My question is: do we have anything in a DFT calculation that is indicative of a multireference system?




You should have a look at the so called FOD (Fermi Occupied Density) analysis by Grimme et al. That is accessible by ORCA 4 very easy and straight forward by specifying "FOD" as a keyword. [1-3]


[![FOD Plot from ORCA Input Library](https://i.stack.imgur.com/dNFJW.jpg)](https://i.stack.imgur.com/dNFJW.jpg)  

From ORCA Input Library [1]:



> 
> The FOD plot above (plotted with Chemcraft) indicates a significant and delocalized FOD. Multiconfigurational methods should be used to study this molecule and any single-reference result (HF, DFT, MP2, CC should be regarded with suspicion)
> 
> 
> 




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Via Multiwfn you can access the "local total/dynamic/nondynamic electron correlation function" and "total/dynamic/nondynamic electron correlation index" [4,5].


[![Local Descriptors of Dynamic and Nondynamic Correlation from [5]](https://i.stack.imgur.com/3FvrT.gif)](https://i.stack.imgur.com/3FvrT.gif)  

Visual abstract from [5]. You can see the dynamic and non-dynamic correlation from which you also can see if single reference methods are acceptable or not.




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[1] [ORCA Input Library, FOD analysis](https://sites.google.com/site/orcainputlibrary/orbital-and-density-analysis/fod-analysis)  

[2] [S. Grimme, A. Hansen, *Angew. Chem. Int. Ed.* **2015**, *54* (42), 12308-12313](http://dx.doi.org/10.1002/anie.201501887)  

[3] [C. A. Bauer, A. Hansen, S. Grimme, *Chem. Eur. J.* **2017**, *23* (25), 6150-6164](http://dx.doi.org/10.1002/chem.201604682)  

[4] [E. Ramos-Cordoba, P. Salvadorc, E. Matito, *Phys. Chem. Chem. Phys.* **2016**, *18*, 24015-24023](http://dx.doi.org/10.1039/C6CP03072F)  

[5] [E. Ramos-Cordoba, E. Matito, *J. Chem. Theory Comput.* **2017**, *13* (6), 2705–2711](http://dx.doi.org/10.1021/acs.jctc.7b00293)


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