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## Main Question or Discussion Point

In Configuration Interaction scheme Why does inclusion of excited states

(unfilled states) in the basis set improve accuracy? What is missing in the model which is accounted for by inclusion of suc h virtual states. My guess is the the born-oppenheimer approximation but not sure??

(unfilled states) in the basis set improve accuracy? What is missing in the model which is accounted for by inclusion of suc h virtual states. My guess is the the born-oppenheimer approximation but not sure??