Recently I have been dealing with a lot of transition states and relatively loosely bound ion-dipole complexes and I have some trouble figuring out how to make sure that the rotational and vibrational contributions to the Gibbs free energy are as accurate as they reasonably can. (I am using Gaussian 09 rev C)
I am using MP2/aug-cc-pVTZ, based on literature results it should be in close agreement with even better methods for the type of molecules I am working with.
First I have optimized the geometry to verytight convergence, then reoptimized it with verytight, CalcAll to make sure the geometry is as converged as possible, and get the thermochemistry output at the same time.
I have once read in a CCL post, that tightening SCF convergence can increase the accuracy of analytic derivatives, so I am performing all calculations with SCF=Conver=12.
My concerns are mostly related to having a number of low frequency modes, and the possibility of coupling between vibrations and rotations.
For example, in the case of the transition state between $\ce{MeCl + F-}$ and $\ce{MeF + Cl-}$, I have three low frequency modes: 258.4867 $cm^{-1}$, 258.4867 $cm^{-1}$ 303.2302 $cm^{-1}$ (and of course an imaginary frequency). Sure enough, Gaussian warns that:
Warning -- explicit consideration of 3 degrees of freedom as vibrations may cause significant error
Based from what I gathered from various sources, this means that these low frequencies have significant uncertainties in them, and they may or may not be internal/hindered rotations. I have ran a calculation with Freq=hindrot but it did not identify any of the modes as rotations. However, the Gaussian manual says that the hindered rotor analysis is not always successful at finding rotations when transition states are involved.
My other concern is the validity of the rigid rotor approximation for molecules having low frequency modes (ie. low force constants). My intuitions tell me, that the lower the force constants in a molecule, the worse are the errors caused by the centrifugal distortions of the molecule.
So my actual questions are:
1.What is the proper way to accurately handle low frequency modes? How does one make sure that they are not some sort of rotation, when hindered rotor analysis does nothing? If they legit low frequency vibrations, are they reliable? If they are not reliable how can they be corrected?
2.Do I have to be concerned about low frequencies also implying that the rotational contributions are also affected due to centrifugal distortion?