- #1
AxiomOfChoice
- 533
- 1
The spectral theorem states that if [itex]\hat H[/itex] is a COMPACT self-adjoint operator on a Hilbert space [itex]\mathcal H[/itex], there is a basis for [itex]\mathcal H[/itex] consisting of of eigenvalues of [itex]\hat H[/itex]. But how are we supposed to determine if a given Hamiltonian is compact? For example, in many introductions to the Born-Oppenheimer approximation, the claim is made that the electron Hamiltonian
[tex]
\hat H_e = -\sum_{i = 1}^M \frac{\hbar^2}{2m_e} \Delta_{r_i} + V_{ee} + V_{eN} + V_{NN}
[/tex]
is self-adjoint and therefore has a basis of eigenvalues. (The three V's above are the coulombic nuclear-nuclear, nuclear-electron, and electron-electron interactions.) That [itex]\hat H_e[/itex] is self-adjoint is pretty obvious...but how do we know it's compact?
[tex]
\hat H_e = -\sum_{i = 1}^M \frac{\hbar^2}{2m_e} \Delta_{r_i} + V_{ee} + V_{eN} + V_{NN}
[/tex]
is self-adjoint and therefore has a basis of eigenvalues. (The three V's above are the coulombic nuclear-nuclear, nuclear-electron, and electron-electron interactions.) That [itex]\hat H_e[/itex] is self-adjoint is pretty obvious...but how do we know it's compact?