Why does the computed energy of a set of semi-free atoms deviate from analytic/experimental values?

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Forum Regular

6:15:08 AM PDT - Thu, May 28th 2020
In short, your level of theory (method + basis set) is insufficient to achieve the level of agreement that you seek. Also, one atom would be sufficient for what you are trying to do, unless you are also trying to test how far apart the atoms need to be in order for them to be (effectively) non-interacting.