Subject: internal rotations 'disguised' as vibrations

From: Patrick Bultinck <[email-masked]>
Date: Mon, 16 Oct 2000 16:37:45 +0100 (NFT)

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