How Do You Compute the 1st Order Wave Function Correction in Quantum Mechanics?

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SUMMARY

The discussion focuses on computing the first-order wave function correction in quantum mechanics using perturbation theory, specifically in the context of an infinite potential well. The formula for the first-order correction is given as ψ_n^{(1)} = Σ_{l ≠ n} <ψ_n^{(0)|H'|ψ_l^{(0)}> / (E_n^{(0)} - E_l^{(0)}) ψ_l^{(0)}. It is established that if the perturbation H' is a constant, there is no first-order correction since it does not alter the eigenstates of the Hamiltonian. The key takeaway is the importance of identifying non-zero terms in the summation for analytical evaluation.

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Homework Statement



A have a bit of a general question regarding 1st order wave function corrections using perturbation theory.

In a problem like the infinite potential well where you have states numbered like n = 1, 2, 3, ..., how do you compute the sum for the 1st order correction when you have infinite terms?:

\psi_n^{(1)} = \Sigma_{l \ne n} \frac{&lt;\psi_n^{(0)}|H&#039;|\psi_l^{(0)}&gt;}{E_n^{(0)} - E_l^{(0)}} \psi_l^{(0)}

I guess I don't know how to get <n|H'|l> so I can evaluate the sum
 
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Do you know what your perturbation H' looks like? Sometimes the non-zero terms in the summation result in something that can be summed analytically.
 
This was the thinking I was missing!

So for H' = constant there is no first-order correction because l \ne n, yes?
 
Correct. If you add a constant to your Hamiltonian, you shift the zero of energy but you do not change its eigenstates.
 

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