Hi all. I m stucked on the question followed, any helps will be greatly appreciated. A perturbation has the form H'=z act on a two level system which they have different parity. So the first order correction to the energy level 1 and 2 are give by: E11=<ψ1| H' |ψ1> Same for level 2. These two energy correction are clearly zero because the same level has same parity. However the first order wavefunction corrections are not for both level: According to time independent non-degenerate perturbation theory on the 1st order correction to the wave function formula: ψ11=<ψ2| H ' | ψ1>/(E1-E2) ψ2 Which is none zero Where am I wrong? I am so confused..thanks in advance.