From owner-chemistry.,at,.ccl.net Thu Dec 20 03:17:00 2007 From: "Philippe Carbonniere philippe.carbonniere[]univ-pau.fr" To: CCL Subject: CCL:G: "Low frequencies" in Gaussian Freq Jobs Message-Id: <-35889-071219123617-1908-cISwCrpdtj1+sFWQsg6kXg-x-server.ccl.net> X-Original-From: Philippe Carbonniere Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1; format=flowed Date: Wed, 19 Dec 2007 17:35:56 +0100 MIME-Version: 1.0 Sent to CCL by: Philippe Carbonniere [philippe.carbonniere],[univ-pau.fr] Dear Andrea, Pay attention to use opt=3Dtight (but maybe this is the default for this = kind of calculations) and if you use DFT : int=3Dgrid=3Dultrafine. You wi= ll=20 lower the six eigenvalues corresponding to rotation and translation. Concerning the common range of this values I would say between -5 and=20 +(5-10) cm-1 for organic systems. Maybe this changes for inorganic=20 systems (who knows !) but I have no experience on that systems. Hopping this will be help you, Philippe Carbonniere Andrea Ciccioli andrea.ciccioli+*+uniroma1.it a =E9crit : >Sent to CCL by: "Andrea Ciccioli" [andrea.ciccioli_-_uniroma1.it] >Dear friends, > >it is commonly asserted in textbooks and software manuals that the obvio= us test to recognize minima in PES among stationary points is that the vi= brational frequencies have to be real. >However, I wonder if besides this criterium one should also check carefu= lly the values of the frequencies reported as "Low frequencies" in Gaussi= an outputs (just before the list of Harmonic frequencies). These are the = "frequencies" actually corresponding to translations and rotations, and t= hey should be ideally equal to zero, and indeed in many cases they are ve= ry low. However, it happens not seldom to me, e.g. for triatomic species = containing heavy elements such as transition metals, to obtain outputs wh= ere, although the harmonic vibrational frequencies are all real (positive= numbers in the Gaussian output), ie the structure should be a minimum, n= evertheless one or two "Low Frequencies" are not that low. Furthermore, t= hey are in general both positive and negative. For example, low frequenci= es as high as +/- 10 to 40 cm-1 are obtained. Moreover, these values are= apparently larger for analytic second-derivative frequency calculations = than for numerical calculations (I use DF! > T methods). >As far as you know, these relatively high values of the "Low Frequencies= " can indicate that the calculated structure is not a true minimum, in sp= ite of having real harmonic frequencies ? What could be a reasonable crit= erium to consider the "Low frequencies" small enough to be sure that the = stationary point is a true minimum ? >Has anyone some suggestions/indications to give me ? >Thanks to all, and season's greetings. >Andrea Ciccioli >University of Rome (ITALY) >Sapienza > > > >-=3D This is automatically added to each message by the mailing script =3D= -=> >Subscribe/Unsubscribe:=20> >Job: http://www.ccl.net/jobs=20> >Search Messages: http://www.ccl.net/htdig (login: ccl, Password: search= )> > > > =20 >