From chemistry-request@ccl.net Thu Jul 22 01:04:23 2004 Received: from relay4.delfa.net (relay4.delfa.net [193.125.210.9]) by server.ccl.net (8.12.8/8.12.8) with ESMTP id i6M64LZf005746 for ; Thu, 22 Jul 2004 01:04:22 -0500 X-Envelope-To: Received: from sycorax.delfa.net (relay4.delfa.net [193.125.210.9]) by sycorax.delfa.net (8.13.0/8.13.0/Sycorax.Delfa) with ESMTP id i6M6AwgV032670 for ; Thu, 22 Jul 2004 10:10:58 +0400 Received: (from uucp@localhost) by sycorax.delfa.net (8.13.0/8.12.9/Submit) with UUCP id i6M6AwlY032669 for chemistry|at|ccl.net; Thu, 22 Jul 2004 10:10:58 +0400 Received: from goblin ([192.168.5.1] helo=xenon.spb.ru ident=dima) by shadow with esmtp (Exim 3.35 #1 (Debian)) id 1BnWbW-00085d-00 for ; Thu, 22 Jul 2004 09:58:54 +0400 Message-ID: <40FF579E.1060608|at|xenon.spb.ru> Date: Thu, 22 Jul 2004 09:58:54 +0400 From: Dmitry Rozmanov User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.6) Gecko/20040528 Debian/1.6-7 X-Accept-Language: ru, en MIME-Version: 1.0 To: CCL Subject: Re: CCL:force constants of diatomics in GAUSSIAN-03 References: <40F5307B.1020909|at|xenon.spb.ru> <40FC1BDF.1010700|at|xenon.spb.ru> <40FD869D.4060002|at|xenon.spb.ru> <40FE5064.9030808|at|xenon.spb.ru> <20040721142403.GC18470|at|svega.gaussian.com> In-Reply-To: <20040721142403.GC18470|at|svega.gaussian.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, hits=0.9 required=7.5 tests=MY_BAD_DOT autolearn=no version=2.61 X-Spam-Checker-Version: SpamAssassin 2.61 (1.212.2.1-2003-12-09-exp) on servernd.ccl.net Dear Mike, Finaly I feel myself much better as I get the information from the very first hands. Thank you for answering me. I have read this explanation before. I get the point that is the way Gaussian calculates the force constants or whatever it is. But my point is about something different, I think. When people calculate something then they want to discuss and to compare. I tried other QC software and searched the Net for the data to be compared with my calculation. I got some numbers which agree to each other. Ok. Then I wanted to compare the data with the results I got using Gaussian. And ooops! No accord! By the way, the units are the same and no Warning at all. Is that ok? I highly doubt this. I am ready just to accept the fact I need to convert the gaussian's force constants to whole-other-world force constants. Is there any other white paper with a simple step-by-step calculation how to get the "normal" force constant > from the result of a calculation performed by Gaussian? Should I use the factor > from differently calculated reduced masses? Thank you for the comment. Anyway, this is weird... With best wishes. ---Dmitry. Michael Frisch wrote: > On Wed, Jul 21, 2004 at 03:15:48PM +0400, Dmitry Rozmanov wrote: > Dmitry Rozmanov wrote > >>If this is the case, then I guess this is just a wrong way of doing things >>and the force constants got by Gaussian are not correct at all. There is a >>definition of the thing and there is no two way of calculation. >> >>---Dmitry. >> > > > This is nonsense. The details are in a white paper on our web site, > but the key point is that the force constant is the second derivative > with respect to a normal mode displacement and the units for the > normal mode, or equivalently the convention for what consititues a > unit step, are arbitrary. > > For polyatomic molecules, one typically diagonalizes the force > constant matrix in mass-weighted coordinates, so the natural unit step > is a normalized displacement in these coordinates. This approach is > general and applicable to any polyatomic molecule. In the particular > case of H2 with the molecule along the x-axis, this normalized step > would be (1/sqrt(2),0,0,1/sqrt(2),0,0) in the 6 cartesian coordinates. > This unit step changes the H-H distance by sqrt(2). For the particular > case of diatomic molecules when people calculate by hand, they use the > distance between atoms as the coordinate, which simpler for diatomics > but doesn't apply to polyatomics. In that coordinate system, a unit > displacement changes the distance by 1 rather than sqrt(2), so the force > constants (second derivatives of the energy) differ by a factor of 2. > The corresponding reduced masses for the mode also differ by a factor > of two and the frequency is the same. > > For the diatomic, the "by hand" coordinates give a reduced mass for > the mode which is the same as the overall reduced mass for the > molecule. For a general polyatomic molecule, the reduced mass > corresponding to a particular mode is not an observable quantity and > is not defined until one adopts a convention for the (arbitrary) size > of a unit normal mode displacement. > > Mike Frisch > > > > > > -= This is automatically added to each message by the mailing script =- > To send e-mail to subscribers of CCL put the string CCL: on your Subject: line > and send your message to: CHEMISTRY|at|ccl.net > > Send your subscription/unsubscription requests to: CHEMISTRY-REQUEST|at|ccl.net > HOME Page: http://www.ccl.net | Jobs Page: http://www.ccl.net/jobs > > If your mail is bouncing from CCL.NET domain send it to the maintainer: > Jan Labanowski, jlabanow|at|nd.edu (read about it on CCL Home Page) > -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > > > > > >