From owner-chemistry ^%at%^ ccl.net Thu Dec  1 15:57:00 2011
From: "Carlos T Nieto eneas:-:usal.es" <owner-chemistry---server.ccl.net>
To: CCL
Subject: CCL: Additional imaginary frecuencies: tested almost everything
Message-Id: <-45959-111201154843-4582-M1PxjFsrfP+cd2wt04BJcw---server.ccl.net>
X-Original-From: "Carlos T  Nieto" <eneas{=}usal.es>
Date: Thu, 1 Dec 2011 15:48:41 -0500


Sent to CCL by: "Carlos T  Nieto" [eneas]_[usal.es]
Hi, everybody
>
> One of the typical problems in a optimization is the appearrance of
> additional imaginary frecuencies in optimization or transition state
> minimizations.
>
> Im an user of Jaguar. In many frequency calculations (DFT-B3LYP) i obtain small negative frequencies, e.g. <
> -30 cm-1.It's very frequently i obtained it in:
>
>     - Solvent calculations
>     - Transition states when freezing atoms involved in it. It's to
> optimize the different substituents.
>
> When this frequencies are associated with certain type of easy
> identifiable displacement, i modify the geometry along the suspected
> vibration and reoptimize. This works well. Nevertheless, the most
> difficult cases is when the displacement is difficult (some type of small
> apparent rotation where almost all atoms participate) i don't know how to
> disturb it. It often appears when i freeze atoms, let's say to block the
> atoms involved in a TS to add substituents and reoptimize or in solvent
> calculations.
>
> With this type of frequencies i've tried several ways, but inefficiently:
>
>  - Disturbing bulky groups and reoptimizing.
>  - Some authors suggest it's a problem of the grid build up of the dft
> process. I tried to change to fine or ultrafine type grids without success.
>  - Other opinion is that is a mathematical artefact and such frequencies
> could be ignored. I've cheked a few publications and it's indicated.
>  - In Transition state searchs, the small negative frequency is the
> eigenvector n 2. Trying to force to search along eigenvector 1 or lowest
> eigenvector doesn't works again.
>
> So, What else can i do? I'm not sure if i'm doing something wrong... Is
> there a way to getting rid of it? If not, May i accept the result and
> ignore the small imaginary frequency? Other authors said that, accepting
> this, it's not accurate taking the thermodinamic data from the frequency
> calculations...
>
> Will be reportable my results ignoring this additional frecuencies? Shall
> I indicate them?
>
> Ill appreciate all the ideas from you.>