CCL:G: G09: Convergence Issues for H-Bonded Systems

[Bradley Welch <bwelch5||slu.edu>]

 Have you optimized the system at a lower level of theory and have had success?
 


Bradley Welch

On Thu, Sep 22, 2011 at 5:08 PM, Sten Nilsson Lill stenil-x-chalmers.se <owner-chemistry*ccl.net> wrote:

Sent to CCL by: Sten Nilsson Lill [stenil||chalmers.se]
Sam,

in similar cases I've been able to reach convergence by using the keywords scf=novaracc and opt=(tight,newton). To those opt-keyword you could also add calcfc as you mention.

Another aspect of this is the sensitivity of the M06 functionals to the choice of integration grid. It's recommended to use a large grid so if you are using Gaussian you can try Grid=ultrafine. Remember that it will be more costly and that you need to do all your calculations with the same grid settings when comparing energies.

You can read more on this in Houk's JCTC paper from 2010: http://pubs.acs.org/doi/abs/10.1021/ct900639j and some references therein, for example some work by DiLabio and Johnson.

Hope that helps,

Sten

Ph.D. Sten Nilsson Lill
Dep. of Chemistry
University of Gothenburg
SE-412 96 Gothenburg, Sweden
e-mail: stenil^-^chem.gu.se
Phone: +46-31 786 9103
Fax: +46-31-772 3840
________________________________________
> From: owner-chemistry+stenil==chem.gu.se^-^ccl.net [owner-chemistry+stenil==chem.gu.se^-^ccl.net] On Behalf Of Sam Abrash sabrash]_[richmond.edu [owner-chemistry^-^ccl.net]
Sent: Thursday, September 22, 2011 5:25 PM
To: Nilsson Lill, Sten
Subject: CCL: G09: Convergence Issues for H-Bonded Systems

Sent to CCL by: "Sam   Abrash" [sabrash_+_richmond.edu]
Hi Folks,

I have a job involving hydrogen bonding of 3 acetylenes to the benzene cation.  I'm having trouble with the geometric convergence.  I was able to get the system to converge with normal convergence criteria, opt=calcall, and MaxStep=5, but then I got an imaginary frequency of -1.9 wavenumbers.

Subsequently, I changed to Opt=tight, tried both calcfc and calcall, and have steadily reduced MaxStep to 1, but it still won't converge.  The force constant criteria have been met, but the problem is the displacement criteria.  Both this observation and looking at the structures with JMOL show that the problem is finding the minimum in a very shallow potential.

Two questions.  First, is the 1.9 wavenumber imaginary frequency real or an artifact?  Second, any advice on how to force the system to converge?  Model chemistry is M062X/6-311++G**.

Thanks!

Samhttp-:-//www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp://www.ccl.net/chemistry/sub_unsub.shtmlhttp://www.ccl.net/spammers.txt

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