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I am by no means sure, but I think you should write mediep2[i_, x_] := -\!\( \*SubsuperscriptBox[\(\[Integral]\), \(0\), \(\[Infinity]\)]\(\[Psi][ i, x]*\((\[HBar]^2\ )\) D[D[\[Psi][i, x], x], x] \[DifferentialD]x\)\)...