CCL: Vibrational sublevels along particular coordinate on PES

Dear computational chemistry specialists,

Thank you all for your help and clarifications! Now I think I have an idea about analysing a multidimentional PES, at least from the viewpoint of statistical mechanics...

Several minor questions, though:

1.
Eric Hermes erichermes+/-gmail.com wrote: 
Second, this approach is only valid if your reaction coordinate
 is sufficiently flat that it can
 be treated classically.
How flat is "sufficiently flat" PES? For example, I have barriers of 10 to 20 kcal/mol for going from 10° to 90° torsion (actually this is called internal rotation, I think). Is that "sufficiently flat"? Could someone provide examples for "not sufficiently flat" case (I mean, among well-known ones)? Do I understand it correctly that the restriction applies to cases when tunneling appears, which has also Heribert mentioned (as with proton transitions)?

Can I assume that if the PES is "sufficiently flat", then the vibrational sublevels are close to each other? For the my case of internal rotation, for instance...

2.
Heribert Reis hreis]|[eie.gr wrote:
 
the vibrational wave function of the lowest level (for
 simplicity) around that dihedral value
I am not sure I have understood this. In accord with the following discussion, does that mean I would need to solve for the one-dimensional problem with my 1D-PES as the well potential, which would provide me with vibrational levels, of which some will be located at the vicinity of torsion angle value NN°, some other level at MM°, etc., etc.?

Thank you all again in advance!