Sorry for my late reply.
I always perceive the dihedral fitting as the least laborious part of a parameterization project. For the water interactions, one must submit several interaction calculations, with a requirement to think about the right water placement for each of them. For the hard degrees of freedom, putting together a molvib input is significant work. But for the scans, both Gaussian and Q-Chem can do a whole Potential Energy scan with just one input. And the fitting is usually
relatively straightforward as well (though this might be one of the exceptions).
But to answer your question: tropone has a very low normal mode (53 cm-1
at MP2/6-31G(d)) that roughly corresponds to an envelope distortion, ie. the most apparent motion when visualizing this mode is the carbonyl carbon (and oxygen) moving out of the plane of the ring. This mode is important because its very low frequency makes the ring very flexible; indeed, in the colchicine model compound O-methyltropolone, the ring minimizes to a planar or a nonplanar minimum depending on the initial C-C-O-C torsion. These kind of relatively large conformational changes associated with low modes tend to be relatively poorly reproduced when just fitting molvib, so it would be advisable to perform a concerted scan along this mode on tropone before moving on to methyltropolone and scanning the C-C-O-C torsion. I was going to send you an existing example of such a concerted scan, but the examples I found were not optimally relevant for the current problem at hand, so I instead made the attached tropone input.
It should be noted that the 2 dihedrals I'm using to drive the scan are not the only ones that change significantly during the scan; it's geometrically impossible to change less than 4 dihedrals in a ring without big changes in bond lengths. But since this is a relaxed scan, I just let the other dihedrals adapt. I have reasons to believe that the resulting concerted motion in this particular case will closely mimic the actual low mode (and that's more than sufficient for the purpose of dihedral fitting). Conversely, I don't think we could get away with driving the ring scan by only one dihedral because the resulting motion would be qualitatively different from the one we're interested in.
Finally, purely based on considerations of the degree of freedom, this one scan would not be sufficient to fully capture the flexing behavior of this ring. However, given our choice of atom types, I think we will get away with it, especially if we also consider the molvib and/or the conformations that are visited during the scan.