internal rotations 'disguised' as vibrations

[Patrick Bultinck <Patrick.Bultinck $#at#$ rug.ac.be>]

 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