# internal rotations 'disguised' as vibrations

Dear,
When carrying out calculations of thermodynamic parameters for
metallo-organic species, I often get warnings in G98 output : Explicit
consideration of x modes of freedom as vibrations may cause significant
error. Looking at these modes I see they look like internal rotations. I
believe these modes should be subtracted from the vibrational
contribution to the thermal energy,
but they should then be introduced in the rotational partition function,
shouldn't they ? My question is : how do you more experienced
computational chemists introduce them ? I know that there are formula for
special cases, such as symmetric tops with two equal parts, or molecules
with more symmetric tops attached to a rigid frame etc. My question is :
is there a general way to remove these pseudo-vibrations, AND then to add
them in some way to the partition function of rotation (where I feel they
should belong).
My question originates from a few complexes where I have very low modes,
10 cm^-1, where I see these 'vibrations' are in fact internal rotations.
Best regards and thanks,
Patrick Bultinck
Ghent University
Belgium