The Student Room Group

Eigenvalues changing under a change in the potential

If I have computed the eigenvalues of the Hamiltonian (i.e. the allowed energies) for the potential V(x)=12mω2x2 V(x) = \dfrac{1}{2}m\omega^2x^2 to be En=ω(n+12)E_n = \hbar\omega(n+\dfrac{1}{2}) then if the potential is changed to V(x)=12mω2x2+Kx V(x) = \dfrac{1}{2}m\omega^2x^2 + Kx for some real K then am I correct in thinking that the allow energies won't change because we can complete the square and obtain a potential of the same form (translated) plus some constant. But constants in the energy are arbitrary (?) and so are translations so they won't change the energies?

Thanks
Original post by Ryanzmw
If I have computed the eigenvalues of the Hamiltonian (i.e. the allowed energies) for the potential V(x)=12mω2x2 V(x) = \dfrac{1}{2}m\omega^2x^2 to be En=ω(n12)E_n = \hbar\omega(n \dfrac{1}{2}) then if the potential is changed to V(x)=12mω2x2Kx V(x) = \dfrac{1}{2}m\omega^2x^2 Kx for some real K then am I correct in thinking that the allow energies won't change because we can complete the square and obtain a potential of the same form (translated) plus some constant. But constants in the energy are arbitrary (?) and so are translations so they won't change the energies?

Thanks


this isn't quite right, the energies themselves will be different but what will not change is the overall behaiviour of the systen.

i haven't worked out the schrodinger equation fully for this examole but following as per your example you will have a potential of the form (ax b)^2 V_0 via completing the square, where V_0 is a constant offset.

it can be shown that adding a constant offset to the potential only translates the eigenvalues as expected (the only difference is that your wavefunction picks up an extra exponential phase factor). ignoring the phase offset, now you will be solving the schrodinger equation for a slightly modified potential (ax b)^2. the procedure is the same but i don't think the energy eigenvalues will be exactly the same anymore due to the linear scaling (admittedly i havent tried this myself so could have gone wrong somewhere)
(edited 5 years ago)
Reply 2
Original post by silentshadows
this isn't quite right, the energies themselves will be different but what will not change is the overall behaiviour of the systen.

i haven't worked out the schrodinger equation fully for this examole but following as per your example you will have a potential of the form (ax b)^2 V_0 via completing the square, where V_0 is a constant offset.

it can be shown that adding a constant offset to the potential doesnt change the eigenvalues (the only difference is that your wavefunction picks up an extra exponential phase factor). ignoring the phase offset, now you will be solving the schrodinger equation for a slightly modified potential (ax b)^2. the procedure is the same but i don't think the energy eigenvalues will be exactly the same anymore due to the linear scaling (admittedly i havent tried this myself so could have gone wrong somewhere)


I'm confused as to why, ignoring the phase, the solutions won't be the same. If they're defined over all of the real life and the potential is effectively just translated by a finite distance the eigenfunctions won't just have the same translation but the same energies as before.
Original post by Ryanzmw
I'm confused as to why, ignoring the phase, the solutions won't be the same. If they're defined over all of the real life and the potential is effectively just translated by a finite distance the eigenfunctions won't just have the same translation but the same energies as before.


sorry i made a slight error, the constant V_0 term does shift the energies but its just by a translation so we can ignore this for the time being. if you want a more mathematical explanation, we can introduce the new variable x' = ax+b and solve the schrodinger equation in terms of x'. the eigenvalues for x' will be the same, but we want our final answer in terms of the 'standard variable' x meaning we need to scale them back again

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