From chemistry-request.,at,.server.ccl.net Mon Oct 16 10:30:45 2000 Received: from mserv.rug.ac.be (mserv.rug.ac.be [157.193.40.37]) by server.ccl.net (8.8.7/8.8.7) with ESMTP id KAA30562 for ; Mon, 16 Oct 2000 10:30:45 -0400 Received: from hartree4.rug.ac.be (hartree4.rug.ac.be [157.193.91.12]) by mserv.rug.ac.be (8.9.3/8.9.3) with ESMTP id QAA01655 for ; Mon, 16 Oct 2000 16:30:31 +0200 (MET DST) Received: (from patrick.,at,.localhost) by hartree4.rug.ac.be (AIX4.2/UCB 8.7/8.7) id QAA08530; Mon, 16 Oct 2000 16:37:45 +0100 (NFT) Date: Mon, 16 Oct 2000 16:37:45 +0100 (NFT) From: Patrick Bultinck To: chemistry%!at!%ccl.net Subject: internal rotations 'disguised' as vibrations Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII 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