Is my Koopmans theorem calculation for molecule A reasonable?

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SUMMARY

The discussion centers on the application of Koopmans theorem to estimate excitation energy for molecule A. The user calculated the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies for molecule A, A-, and A+. The computed energy gap (Egap) is 1 eV smaller than the experimental value, raising questions about the accuracy of the calculation. The context of the calculation is specified as being in the gas phase.

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greisen
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Hi all,

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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Is this gas phase or solid phase?
 
it is in gas phase
 

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