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I am using Koopmans theorem trying to estimate excitation energy so I have calculated the homo/lumo for molecule A as well as for A- and A+(adding and removing an electron respectively). Then I would use the formula

Egap = min[(E(n+1)-E(n)]-[E(n)-E(n-1)]]

I get a energy gap which is smaller than the experimental value(1 eV less) does this seem like a reasonable computation?

Any advise appreciated - thanks in advance

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