Scripted HTML version of Foils prepared 29 December 1996 
Secs 97.9
| Often you want to find eigenstates |
| where operator H is diagonal with respect to basis set . However, this is usually impossible. |
| Often one knows that |
| for example, if the compound is ,then is the "free" Hamiltonian for isolated molecules |
| and is the interaction, i.e. the forces between atoms. |
| Simple states diagonalize , but will be nonzero for "most" i and j. |
